Make functional module private

[jaraco]

Should this module be private or should importing from importlib_resources.functional be allowed?

Originally posted by @layday in #303 (comment)

[FFY00]

I don't think it should be allowed, that's more exposed API without a real need IMO.

[layday]

This appears to have already been done in the stdlib BTW: https://github.com/python/cpython/blob/main/Lib/importlib/resources/_functional.py. I don't know if any other differences exist.