[predicates] Fix reference to has_related_entities in partial_has_related_entities docstring

Fix possible double import of cubes modules When cubes using the new layout are imported with 'cubicweb_<cube>' and with 'cubes.<cube>', the same module is imported twice. Handle this by adding 'cubes.<cube>' to sys.modules when importing from 'cubicweb_<cube>'. Move load_module() to a sub class _CubesLoader to share informations computed in find_modules(). Don't handle subpackages in _CubesImporter and rely on normal import machinery instead. Add a test and use unittest from cubicweb.devtools.testlib which resolve to unittest2 on PY2 with assertLogs() method.