Dynamic path calculation in PEP 420

I think something along the lines of PJE's approach described in
http://mail.python.org/pipermail/import-sig/2012-April/000473.html is
doable. I think it's somewhat more complex because you need to call
finders to do the path computation, plus there's a cache involved, etc.

I'd be willing to consider include this in the PEP, but I've run out of
time to prove it's feasible by implementing it in the features/pep-420
branch. If anyone else has time, that would be great.

I'm hoping to ask that the PEP approved (or not) by the end of the weekend.

Eric.