[tox] Add a dummy test-results.xml file in check-manifest and flake8 environments
This is to work around Jenkins Junit plugin that does not apparently
account for the absence of this file (though it accepts it to be empty).
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.