[cwconfig] don't rely on _CubeImporter while loading cubes _CubeImporter can load cube subpackages twice (there is a test for submodules in cubicweb.test.unittest_cubes.test_no_double_import() but not with a subpackage). Since _CubeImporter is for backward compatibility and is supposed to be deprecated as soon as possible, don't rely on it when loading cube schema or appobjects and always load the real modname (eg. cubicweb_<cube> if exists else cubes.<cube>).

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