Avoid namespacing routes inside engines

Hello,

Since #11544, invoking the controller generator, any generated route is namespaced according to the class_path method. Since a mountable plugin is namespaced, creating a controller inside would generate a namespaced route based on the engine's name.

The controller generator now relies on regular_class_path which does not contain the class hierarchy but the given path.

cc @rafaelfranca @carlosantoniodasilva

Fixes #14079.

Have a nice day.