Improve scrubbing of namespace packages

Problem: since we can't force python -S using /usr/bin/env, we can't prevent
importing site.  But 'import site' populates sys.modules with placeholder
modules containing .__path__ attributes with site-packages baked in.  This
means even if we scrub sys.path and sys.path_importer_cache, it's still
possible for __import__ to pick up things from site-packages.  This scrubs
sys.modules as well.

Reviewed at https://rbcommons.com/s/twitter/r/116/