scratch. This means they do not set __loader__ by default. This is
acceptable under importlib/PEP 302 definitions, so relax the test that
was trying to apply this universally.
state of the import system. Also make importlib.invalidate_caches()
work with sys.meta_path instead of sys.path_importer_cache to
completely separate the path-based import system from the overall
import system.
Patch by Eric Snow.
