Fix generators to help with ambiguous ApplicationController issue

In development mode, dependencies are loaded dynamically at runtime,
using `const_missing`. Because of that, when one of the constants is
already loaded and `const_missing` is not triggered, user can end up
with unexpected results.

Given such file in an Engine:

```ruby
module Blog
  class PostsController < ApplicationController
  end
end
```

If you load it first, before loading any application files, it will
correctly load `Blog::ApplicationController`, because second line will
hit `const_missing`. However if you load `ApplicationController` first,
the constant will be loaded already, `const_missing` hook will not be
fired and in result `PostsController` will inherit from
`ApplicationController` instead of `Blog::ApplicationController`.

Since it can't be fixed in `AS::Dependencies`, the easiest fix is to
just explicitly load application controller.

closes rails#6413

railties/lib/rails/generators/named_base.rb

@@ -79,6 +79,10 @@ def regular_class_path
           @class_path
         end
 
+        def namespaced_file_path
+          @namespaced_file_path ||= namespaced_class_path.join("/")
+        end
+
         def namespaced_class_path
           @namespaced_class_path ||= begin
             namespace_path = namespace.name.split("::").map {|m| m.underscore }

railties/lib/rails/generators/rails/controller/templates/controller.rb

@@ -1,3 +1,7 @@
+<% if namespaced? -%>
+require "<%= namespaced_file_path %>/application_controller"
+<% end -%>
+
 <% module_namespacing do -%>
 class <%= class_name %>Controller < ApplicationController
 <% actions.each do |action| -%>

railties/lib/rails/generators/rails/scaffold_controller/templates/controller.rb

@@ -1,3 +1,7 @@
+<% if namespaced? -%>
+require "<%= namespaced_file_path %>/application_controller"
+<% end -%>
+
 <% module_namespacing do -%>
 class <%= controller_class_name %>Controller < ApplicationController
   # GET <%= route_url %>

railties/test/generators/namespaced_generators_test.rb

@@ -20,8 +20,14 @@ class NamespacedControllerGeneratorTest < NamespacedGeneratorTestCase
 
   def test_namespaced_controller_skeleton_is_created
     run_generator
-    assert_file "app/controllers/test_app/account_controller.rb", /module TestApp/, /  class AccountController < ApplicationController/
-    assert_file "test/functional/test_app/account_controller_test.rb", /module TestApp/, /  class AccountControllerTest/
+    assert_file "app/controllers/test_app/account_controller.rb",
+                /require "test_app\/application_controller"/,
+                /module TestApp/,
+                /  class AccountController < ApplicationController/
+
+    assert_file "test/functional/test_app/account_controller_test.rb",
+                /module TestApp/,
+                /  class AccountControllerTest/
   end
 
   def test_skipping_namespace
@@ -227,9 +233,10 @@ def test_scaffold_on_invoke
     end
 
     # Controller
-    assert_file "app/controllers/test_app/product_lines_controller.rb" do |content|
-      assert_match(/module TestApp\n  class ProductLinesController < ApplicationController/, content)
-    end
+    assert_file "app/controllers/test_app/product_lines_controller.rb",
+                /require "test_app\/application_controller"/,
+                /module TestApp/,
+                /class ProductLinesController < ApplicationController/
 
     assert_file "test/functional/test_app/product_lines_controller_test.rb",
                 /module TestApp\n  class ProductLinesControllerTest < ActionController::TestCase/