Load Solidus engine extension files automatically

By cleanly separating extension files for the different Solidus
engines, we can only load solidus_backend files when the engine
itself is available rather than doing it all the time.

In turn, this allows extensions to only actually require the parts
of Solidus that they really need.

lib/solidus_support/engine_extensions/decorators.rb

@@ -4,33 +4,68 @@ module SolidusSupport
   module EngineExtensions
     module Decorators
       def self.included(engine)
+        engine.extend ClassMethods
+
         engine.class_eval do
-          extend ClassMethods
           config.to_prepare(&method(:activate).to_proc)
+
+          enable_solidus_engine_support('backend') if SolidusSupport.backend_available?
+          enable_solidus_engine_support('frontend') if SolidusSupport.frontend_available?
+          enable_solidus_engine_support('api') if SolidusSupport.api_available?
         end
       end
 
       module ClassMethods
         def activate
-          base_path = root.join('app/decorators')
-
           if Rails.respond_to?(:autoloaders) && Rails.autoloaders.main
-            # Add decorators folder to the Rails autoloader. This
-            # allows Zeitwerk to resolve decorators paths correctly,
-            # when used.
-            Dir.glob(base_path.join('*')) do |decorators_folder|
+            # Add decorators folder to the Rails autoloader. This tells Zeitwerk to treat paths
+            # such as app/decorators/controllers as roots.
+            solidus_decorators_root.glob('*') do |decorators_folder|
               Rails.autoloaders.main.push_dir(decorators_folder)
             end
           end
 
-          # Load decorator files. This is needed since they are
-          # never explicitely referenced in the application code
-          # and won't be loaded by default. We need them to be
-          # executed anyway to extend exisiting classes.
-          Dir.glob(base_path.join('**/*.rb')) do |decorator_path|
+          load_solidus_decorators_from(solidus_decorators_root)
+        end
+
+        # Loads decorator files.
+        #
+        # This is needed since they are never explicitly referenced in the application code and
+        # won't be loaded by default. We need them to be executed regardless in order to decorate
+        # existing classes.
+        def load_solidus_decorators_from(path)
+          path.glob('**/*.rb') do |decorator_path|
             require_dependency(decorator_path)
           end
         end
+
+        private
+
+        # Returns the root for this engine's decorators.
+        #
+        # @return [Path]
+        def solidus_decorators_root
+          root.join('app/decorators')
+        end
+
+        # Enables support for a Solidus engine.
+        #
+        # This will simply tell Rails to load lib/controllers/[engine] and lib/views/[engine], as
+        # well as loading the decorators from lib/decorators/[engine].
+        #
+        # @see #load_solidus_decorators_from
+        def enable_solidus_engine_support(engine)
+          paths['app/controllers'] << "lib/controllers/#{engine}"
+          paths['app/views'] << "lib/views/#{engine}"
+
+          engine_context = self
+          config.to_prepare do
+            engine_context.instance_eval do
+              path = root.join("lib/decorators/#{engine}")
+              load_solidus_decorators_from(path)
+            end
+          end
+        end
       end
     end
   end