By default, setuptools will discard source distributions which it downloads
after using
them to build and install a binary egg.  For repoze.project, we should figure
out how
to do the followng:

 - Instead of downloading them into t tmpdir, put them into the 'src' directory
   (or something) of the virtualenv.  It should still be able to unpack into a
   tmpdir.

 - Prevent setuptools from deleting the tarball after unpacking it.

If we do this, we can then introspect 'easy_install.pth' to get the
"known good set" for the virtualenv, and even generate a
"package index in a box" from it.