This changes the assumption that these tasks are being run in the rails
root to explicitly determine the binstub directory based on the Rails
root.

This fixes the issue for when the tasks are being run from a mounted
dummy app as part of a Rails engine.

Given a fresh Rails plugin: `rails plugin new rails-plugin --mountable`
that has webpacker gem installed.

Before this change running running `app:webpacker:check_binstubs` would
check in `./bin` for webpack binstubs whereas they are actually expected
in `test/dummy/bin`.

After this change the binstubs for the rails app in `test/dummy/bin` are
checked as expected.

If you have a scenario where a Rails apps tasks are placed under a
namespace these tasks that invoke other tasks don't work.

The common scenario where this happens is when a Rails engine is mounted
in a dummy app as per: https://github.com/rails/rails/blob/master/railties/lib/rails/tasks/engine.rake#L4-L17

This change resolves the issues where you receive an error of task not
found when running an app command e.g. app:assets:precompile.

If we are able to know a task dependency prior to runtime we don't have
to do anything complex as per the change for `clobber` since it being
defined outside a block does not require a namespace.

However for the tasks that we need to perform actions at runtime we have
to work out the prefix of the task that was executed to then re-use this
on the tasks we want to execute.

When this gem is running inside a Rails engine to do a task such as
`app:assets:precompile` an error is thrown:

```
➜  rails-plugin git:(master) ✗ rake app:assets:precompile
Compiling…
Compilation failed:
/Users/kevindew/dev/webpacker/lib/webpacker/railtie.rb:6:in `<top (required)>': uninitialized constant Rails::Engine (NameError)
	from /Users/kevindew/dev/webpacker/lib/webpacker.rb:38:in `require'
	from /Users/kevindew/dev/webpacker/lib/webpacker.rb:38:in `<top (required)>'
	from /Users/kevindew/dev/rails-plugin/test/dummy/bin/webpack:13:in `require'
	from /Users/kevindew/dev/rails-plugin/test/dummy/bin/webpack:13:in `<main>'
```
Explicitly requiring rails/engine resolves this.

This resolves problems that occur when the expectation is that the
Rails.root is the same as the working directory of the rakefile.

A common situation for this is when you have a mounted Rails engine in
the test/dummy app to run your engine.

This changes the yarn commands to run in the directory where you would
have the package.json for the rails app that would be used for mounting
rather than the root of the engine. And the node_modules are also
installed within that directory so they are in the expected place for
tasks such as app:assets:precompile.

The contents of the gitignore only really make sense for the root
directory of a Rails application. If we are dealing with a Rails engine
that mounts the engine onto a test one for usage then it does not make
sense to update the root .gitignore and if someone does have one in the
`test/dummy` directory then that one should be updated instead.

This is resolution to problems that occur with a mounted Rails engine.
With one of these you would want to run `rake app:webpacker:info` and
have it run against the Rails app you'd be mounting in (normally in
test/dummy) however it would incorrectly run in the root directory -
where you probably don't want to have webpacker and webpacker-dev-server
installed.

For a mounted Rails engine we need the config to be in the application
mount directory e.g. test/dummy/config/webpacker.yml

By running this based on pwd we get this in a Rails engine app:

```
➜  rails-plugin rake app:webpacker:verify_install
RAILS_ENV=development environment is not defined in config/webpacker.yml, falling back to production environment
Configuration test/dummy/config/webpacker.yml file not found.
Make sure webpacker:install is run successfully before running dependent tasks
```

Whereas in a normal Rails installation it outputs unchanged:

```
➜  full-app git:(master) ✗ rake webpacker:verify_install
RAILS_ENV=development environment is not defined in config/webpacker.yml, falling back to production environment
Configuration config/webpacker.yml file not found.
Make sure webpacker:install is run successfully before running dependent tasks
```

This sets up a barebones mounted Rails engine so that tests can be
performed against it.

This follows a similar approach to the other rake tasks test and thus
doesn't intend to be too exhaustive in what is testing and is mostly a
cursory test that one of the tasks does perform differently in a mounted
Rails engine context.