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.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)