Clean: Move a comment to where it belongs.
Clean: Compute path to loader modules and initialization scripts globally.
Clean: ImportDirector is not a Owner, so do not inherit it.
Clean: Move constant attribute `path` from instance to class.
Add support for `nspkg.pth`-style namespaces, used by e.g. zope.interface.

This works by
- adding a new NamespaceImportDirector to the end of the meta-path,
- adding a new NamespaceModule module type handling the sub-import,
- faking an empty module for each namespace.

This also includes a testcase for zope.interface.