Vcs unpack once per run

This improves performance dramatically, and ensures that the
dependencies etc do not change while pip-compile runs.

This will clone the repo just once, instead of 4 times:

> % grep "PIP said: Cloning " /tmp/pip-tools.log* -c 
/tmp/pip-tools.log.patched:2
/tmp/pip-tools.log.orig:8