Use require instead of autoload for bundler/plugin/api

This was originally a workaround fix in rubygems#5010, to prevent the following error in fastlane and rdoc on bundler 1.14.3, but the following commit undid this, and this errors is reappearing:
rubygems@6167ec8

```
gems/bundler-1.14.3/lib/bundler/rubygems_ext.rb:45:in `full_gem_path': uninitialized constant Bundler::Plugin::API::Source (NameError)
from /lib/ruby/site_ruby/2.0.0/rubygems/basic_specification.rb:144:in `block in full_require_paths'
```

This commit will revert that change.

lib/bundler/plugin/api.rb

@@ -1,4 +1,7 @@
 # frozen_string_literal: true
+# Should not use `autoload :Source, "bundler/plugin/api/source"` as this breaks sdoc, fastlane with this error:
+# 
+require "bundler/plugin/api/source"
 
 module Bundler
   # This is the interfacing class represents the API that we intend to provide
@@ -23,7 +26,6 @@ module Bundler
   # and hooks).
   module Plugin
     class API
-      autoload :Source, "bundler/plugin/api/source"
 
       # The plugins should declare that they handle a command through this helper.
       #