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 controller_class_path (provided by ResourceHelpers) which is generated according to the given controller name and not the current class hierarchy.

I haven't found any way to write a regression test though ; I don't know if there is an easy way to test this. 

cc @rafaelfranca @carlosantoniodasilva : Yves told me that he was not sure that we should rely on 

Fixes #14079.

Have a nice day.