Avoid namespacing routes inside engines

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 which is
generated according to the given controller name and not the current
class hierarchy.

Fixes #14079.