[Import-SIG] PEP 420 status

[Eric V. Smith]

I think I've addressed all outstanding issues. So if no one disagrees,
I'll ask Guido to rule on it.

The implementation in features/pep-420 is basically complete. I'll
address the remaining issues once it's accepted (if it is).

These issues include:
- more complete tests. The existing tests are pretty good, but there are
  some corner cases that need exercising.
- zipimport needs to work even if there's no "directory" entry for
  a portion.

Eric.