Add support for redirecting for a namespace package.

[jaraco]

This alternative to #35 simply provides redirects for submodules if not found by other finders.

[pfmoore]

I'll need to remind myself of the history here - the checks you're removing were deliberately included, because from what I recall I had strong reservations about the actual meaning of integrating namespace packages into editable wheels.

I don't have a lot of free time this week, so it may be a while before I can review this and come back with more complete feedback - please ping me in a couple of weeks if I've not made any further comments by then.