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.