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 regular_class_path which does not
contain the class hierarchy but the given path.

Fixes #14079.
Merge pull request #14195 from robin850/issue-14079

Avoid namespacing routes inside engines