accept python sdists as deps somehow

[benley]

the eggs attribute works with eggs and wheels, but currently there is no easy way to build a distribution from a python sdist archive.

One of the simplest ways to accomplish this would be to add a find_links attribute that added paths to be searched for requirements.

What might be better is a new rule type (or macro) that converted sdists into wheels, which could then be put into a pex_library. This would result in better build caching.