Make it possible to set StyleGuideBaseURL per department

### Summary

Resolves rubocop/rubocop-rails#107
and follow up rubocop/rubocop-rails#109 (comment).

This PR makes it possible to set a style guide URL (`StyleGuideBaseURL`)
for each department.

For example, RuboCop Rails gem (Rails department) supports
The Rails Style Guide. In that case, set .rubocop.yml as follows.

```yaml
# .rubocop.yml
require:
  - rubocop-rails

AllCops:
  StyleGuideBaseURL:  https://rubystyle.guide

Rails:
  StyleGuideBaseURL:  https://rails.rubystyle.guide
```

In practice `Rails: StyleGuideBaseURL: https://rails.rubystyle.guide`
will be set in config/default.yml of rubocop-rails.

```console
% cat example.rb
# frozen_string_literal: true

time = Time.new
```

```console
% bundle exec rubocop --display-style-guide example.rb
Inspecting 1 file
W

Offenses:

example.rb:3:1: W: Lint/UselessAssignment: Useless assignment to
variable - time. (https://rubystyle.guide#underscore-unused-vars)
time = Time.new
^^^^
example.rb:3:13: C: Rails/TimeZone: Do not use Time.new without
zone. Use one of Time.zone.now, Time.current, Time.new.in_time_zone,
Time.new.utc, Time.new.getlocal, Time.new.xmlschema, Time.new.iso8601,
Time.new.jisx0301, Time.new.rfc3339, Time.new.httpdate, Time.new.to_i,
Time.new.to_f instead. (https://rails.rubystyle.guide#time,
http://danilenko.org/2012/7/6/rails_timezones)
time = Time.new
            ^^^

1 file inspected, 2 offenses detected
```

The style guide URLs set for each department is displayed.

- https://rubystyle.guide#underscore-unused-vars for `Lint/UselessAssignment` cop
- https://rails.rubystyle.guide#time for for `Rails/TimeZone` cop

So `AllCops` style guide URL is used when there is no style guide URL
for a specific department.

### Other Information

Note that tasks/cops_documentation.rake will require the following patch:

```diff
diff --git a/tasks/cops_documentation.rake
b/tasks/cops_documentation.rake
index bd93ffe31..751222fcb 100644
--- a/tasks/cops_documentation.rake
+++ b/tasks/cops_documentation.rake
@@ -155,7 +155,9 @@ task generate_cops_documentation:
:yard_for_generate_documentation do

   def references(config, cop)
     cop_config = config.for_cop(cop)
-    urls = RuboCop::Cop::MessageAnnotator.new(config, cop_config, {}).urls
+    urls = RuboCop::Cop::MessageAnnotator.new(
+      config, cop.name, cop_config, {}
+    ).urls
     return '' if urls.empty?

     content = h3('References')
```

AFAIK, rubocop-performance, rubocop-rails, rubocop-rspec, and
rubocop-minitest repositories are targeted.

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * [#7274](https://github.com/rubocop-hq/rubocop/issues/7274): Add new `Lint/SendWithMixinArgument` cop. ([@koic][])
 * [#7272](https://github.com/rubocop-hq/rubocop/pull/7272): Show warning message if passed string to `Enabled`, `Safe`, `SafeAutocorrect`, and `AutoCorrect` keys in .rubocop.yml. ([@unasuke][])
+* [#7295](https://github.com/rubocop-hq/rubocop/pull/7295): Make it possible to set `StyleGuideBaseURL` per department. ([@koic][])
 
 ### Bug fixes
 

lib/rubocop/config.rb

@@ -96,6 +96,10 @@ def for_cop(cop)
       @for_cop[cop.respond_to?(:cop_name) ? cop.cop_name : cop]
     end
 
+    def for_department(department_name)
+      @for_cop[department_name]
+    end
+
     def for_all_cops
       @for_all_cops ||= self['AllCops'] || {}
     end

lib/rubocop/cop/cop.rb

@@ -224,8 +224,8 @@ def excluded_file?(file)
 
       def annotate(message)
         RuboCop::Cop::MessageAnnotator.new(
-          config, cop_config, @options
-        ).annotate(message, name)
+          config, cop_name, cop_config, @options
+        ).annotate(message)
       end
 
       def file_name_matches_any?(file, parameter, default_result)

lib/rubocop/cop/message_annotator.rb

@@ -9,11 +9,11 @@ module Cop
     #
     # @example
     #   RuboCop::Cop::MessageAnnotator.new(
-    #     config, cop_config, @options
-    #   ).annotate('message', 'Cop/CopName')
+    #     config, cop_name, cop_config, @options
+    #   ).annotate('message')
     #  #=> 'Cop/CopName: message (http://example.org/styleguide)'
     class MessageAnnotator
-      attr_reader :options, :config, :cop_config
+      attr_reader :options, :config, :cop_name, :cop_config
 
       @style_guide_urls = {}
 
@@ -29,6 +29,7 @@ class << self
       #     :ExtraDetails [Boolean] Include cop details
       #     :DisplayCopNames [Boolean] Include cop name
       #
+      # @param [String] cop_name for specific cop name
       # @param [Hash] cop_config configs for specific cop, from config#for_cop
       # @option cop_config [String] :StyleGuide Extension of base styleguide URL
       # @option cop_config [String] :Reference Full reference URL
@@ -43,8 +44,9 @@ class << self
       #   Include debug output
       # @option options [Boolean] :display_cop_names
       #   Include cop name
-      def initialize(config, cop_config, options)
+      def initialize(config, cop_name, cop_config, options)
         @config = config
+        @cop_name = cop_name
         @cop_config = cop_config || {}
         @options = options
       end
@@ -53,8 +55,8 @@ def initialize(config, cop_config, options)
       # based on params passed into initializer
       #
       # @return [String] annotated message
-      def annotate(message, name)
-        message = "#{name}: #{message}" if display_cop_names?
+      def annotate(message)
+        message = "#{cop_name}: #{message}" if display_cop_names?
         message += " #{details}" if extra_details? && details
         if display_style_guide?
           links = urls.join(', ')
@@ -74,7 +76,7 @@ def style_guide_url
         return nil if url.nil? || url.empty?
 
         self.class.style_guide_urls[url] ||= begin
-          base_url = config.for_all_cops['StyleGuideBaseURL']
+          base_url = style_guide_base_url
           if base_url.nil? || base_url.empty?
             url
           else
@@ -83,6 +85,13 @@ def style_guide_url
         end
       end
 
+      def style_guide_base_url
+        department_name = cop_name.split('/').first
+
+        config.for_department(department_name)['StyleGuideBaseURL'] ||
+          config.for_all_cops['StyleGuideBaseURL']
+      end
+
       def display_style_guide?
         (options[:display_style_guide] ||
          config.for_all_cops['DisplayStyleGuide']) &&

spec/rubocop/cop/message_annotator_spec.rb

@@ -3,11 +3,14 @@
 RSpec.describe RuboCop::Cop::MessageAnnotator do
   let(:options) { {} }
   let(:config) { RuboCop::Config.new({}) }
-  let(:annotator) { described_class.new(config, config['Cop/Cop'], options) }
+  let(:cop_name) { 'Cop/Cop' }
+  let(:annotator) do
+    described_class.new(config, cop_name, config[cop_name], options)
+  end
 
   describe '#annotate' do
     subject(:annotate) do
-      annotator.annotate('message', 'Cop/Cop')
+      annotator.annotate('message')
     end
 
     context 'with default options' do
@@ -55,9 +58,10 @@
 
   describe 'with style guide url' do
     subject(:annotate) do
-      annotator.annotate('', 'Cop/Cop')
+      annotator.annotate('')
     end
 
+    let(:cop_name) { 'Cop/Cop' }
     let(:options) do
       {
         display_style_guide: true
@@ -103,6 +107,28 @@
         expect(annotate).to include('http://example.org/styleguide#target_based_url')
       end
 
+      context 'when a department other than AllCops is specified' do
+        let(:config) do
+          RuboCop::Config.new(
+            'AllCops' => {
+              'StyleGuideBaseURL' => 'http://example.org/styleguide'
+            },
+            'Foo' => {
+              'StyleGuideBaseURL' => 'http://foo.example.org'
+            }
+          )
+        end
+
+        let(:cop_name) { 'Foo/Cop' }
+        let(:urls) { annotator.urls }
+
+        it 'returns style guide url when it is specified' do
+          config['Foo/Cop'] = { 'StyleGuide' => '#target_style_guide' }
+
+          expect(urls).to eq(%w[http://foo.example.org#target_style_guide])
+        end
+      end
+
       it 'can use a path-based setting' do
         config['Cop/Cop'] = { 'StyleGuide' => 'cop/path/rule#target_based_url' }
         expect(annotate).to include('http://example.org/cop/path/rule#target_based_url')

tasks/cops_documentation.rake

@@ -155,7 +155,9 @@ task generate_cops_documentation: :yard_for_generate_documentation do
 
   def references(config, cop)
     cop_config = config.for_cop(cop)
-    urls = RuboCop::Cop::MessageAnnotator.new(config, cop_config, {}).urls
+    urls = RuboCop::Cop::MessageAnnotator.new(
+      config, cop.name, cop_config, {}
+    ).urls
     return '' if urls.empty?
 
     content = h3('References')