[cwconfig] create a virtual "cubes" package
_CubesImporter now handle import of "cubes" and return a virtual module so that
"cubes" is always importable without having to create a __init__.py in
CUBES_DIR and add CUBES_DIR/.. to sys.path.
Drop custom code in setup.py used to create CUBES_DIR/__init__.py and drop now
useless warning when cubicweb is installed in develop mode
Update test_cubes_path() and ensure 'cubes.file' is not loaded before testing
its import. This test seems to have mistakes because cubes are loaded
automatically when discovering cw commands (cubes.<x>.ccplugin), not sure how
this should be fixed definitely.