means that people are finally doing enough things with setuptools to
have real-life version conflict scenarios.  Luckily, the fix is trivial:
use breadth-first instead of depth-first dependency processing, which I
thought we were already doing anyway, but weren't.  And we were giving
precedence to already-installed packages, which means upgrades didn't
work so well.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041265