[Import-SIG] Implementation of PEP 420

[Eric V. Smith]

I have created http://bugs.python.org/issue14660 to track the
implementation of PEP 420. The code is checked in to features/pep-420.

The code currently works, but as described in the issue there's still
work to do.

For items related directly to the implementation, I think we should
discuss them on the issue. And continue the PEP discussion here.

Eric.