[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.