Fix installation method as a gem executable

To support `solidus_auth_devise`, it needs to rely on a new version of
`solidus_support` that changes how it detects if a Solidus frontend is
available and the moment it adds the extensions' controller & view
directories to the Rails known paths.

The previous `solidus_support` version added the extensions' paths on
the engine's (in this case, `solidus_auth_devise`'s engine) load time
and only when `SolidusSupport#frontend_available?` returned `true`.
Engines are loaded before the application code, so at that time, the
monkey patching of `#frontend_available?` that we were generating was
not at hand. The solution is two-fold: we add the paths on an
`initializer` on one side. However, at that moment, the application code
is neither available, so on the other side, we change
`#frontend_available?` to look for a
`Rails.configuration.x.solidius.frontend_available` setting that we add
to `application.rb` using the generator code. To understand the whole
picture, keep in mind that `Rails.configuration` is available on an
initializer, but it's not on an engine's load time.
On top of that, we need to update a couple of things on the generated code:

- We need to reference `Rails.root` instead of `Engine.root`, as
  `SolidusStarterFrontend::Engine` is unavailable on this installation
  method.
- We need to copy to `application.rb` the custom code on
  `SolidusStarterFrontend::Engine` for the same reason as above.

app/controllers/concerns/solidus_starter_frontend/auth_views.rb

@@ -10,7 +10,7 @@ module AuthViews
     protected
 
     def configure_views
-      prepend_view_path Engine.root.join('lib', 'views', 'auth')
+      prepend_view_path Engine.join('lib', 'views', 'auth')
     end
   end
 end

lib/generators/solidus_starter_frontend/solidus_starter_frontend_generator.rb

@@ -5,26 +5,39 @@ class SolidusStarterFrontendGenerator < Rails::Generators::Base
 
   def install
     # Copy directories
-    directory 'app', 'app'
-    directory 'lib/views', 'lib/views'
+    directory 'app', 'app', exclude_pattern: /auth_views/
+    directory 'lib/views', 'app/views'
 
     # Copy files
     copy_file 'lib/solidus_starter_frontend_configuration.rb', 'lib/solidus_starter_frontend_configuration.rb'
     copy_file 'lib/solidus_starter_frontend/config.rb', 'lib/solidus_starter_frontend/config.rb'
-    copy_file 'lib/solidus_starter_frontend/solidus_support_extensions.rb', 'lib/solidus_starter_frontend/solidus_support_extensions.rb'
-
-    # Initializer
-    initializer 'solidus_starter_frontend.rb' do
-      "require 'solidus_starter_frontend/solidus_support_extensions'"
-    end
 
     # Routes
     copy_file 'config/routes.rb', 'tmp/routes.rb'
     prepend_file 'config/routes.rb', File.read('tmp/routes.rb')
 
+    # Enable Solidus frontend
+    application do
+      <<~RUBY
+        config.x.solidus.frontend_available = true
+        config.to_prepare do
+          if defined?(Spree::Auth::Engine)
+            [
+              Spree::UserConfirmationsController,
+              Spree::UserPasswordsController,
+              Spree::UserRegistrationsController,
+              Spree::UserSessionsController,
+              Spree::UsersController
+            ].each do |auth_controller|
+              auth_controller.include SolidusStarterFrontend::Taxonomies
+            end
+          end
+        end
+        RUBY
+    end
+
     # Gems
     gem 'canonical-rails'
-    gem 'solidus_support'
     gem 'truncate_html'
 
     # Text updates

lib/generators/solidus_starter_frontend/views/override_generator.rb

@@ -7,7 +7,7 @@ module SolidusStarterFrontend
   module Views
     class OverrideGenerator < ::Rails::Generators::Base
       def self.views_folder
-        Engine.root.join('app', 'views', 'spree')
+        Rails.root.join('app', 'views', 'spree')
       end
 
       VIEWS = Dir.glob(views_folder.join('**', '*'))

lib/solidus_starter_frontend.rb

@@ -6,7 +6,6 @@
 require 'solidus_core'
 require 'solidus_support'
 
-require 'solidus_starter_frontend/solidus_support_extensions'
 require 'solidus_starter_frontend/version'
 require 'solidus_starter_frontend/config'
 require 'solidus_starter_frontend/engine'

lib/solidus_starter_frontend/engine.rb

@@ -9,6 +9,10 @@ class Engine < Rails::Engine
 
     engine_name 'solidus_starter_frontend'
 
+    initializer 'solidus_starter_frontend', before: 'solidus_support_frontend_paths' do
+      Rails.configuration.x.solidus.frontend_available = true
+    end
+
     # use rspec for tests
     config.generators do |g|
       g.test_framework :rspec