Use pkgutil.ImpLoader instead of a custom loader for cubes import redirection
It just works fine (same implementation of load_module method as _CubesLoader
introduced in d404fd8499dd) and is complete w.r.t. PEP 302 (all methods
implemented).
Related to #13001466.
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc