This finally drop support for legacy cubes.

In cwconfig, don't load modules in the "cubes" namespace.
In cube_pkginfo() handle CW_MIGRATION_MAP which wasn't working with new style cube.
Drop all method specific to legacy cubes importer: cubes_search_path(), extrapath(), cls_adjust_sys_path()
Drop environment variables related to legacy cubes: CW_CUBES_PATH and CW_CUBES_DIR