Look for requested pypi package version
Look for requested pypi package version

Old implementation requested only the latest version
for required package, e.g., making impossible installing
kombu==1.0.4 while the latest was 2.5.10.

New implementation loads all available versions and chooses
the latest one that meets existing requirements.

Change-Id: I6618707b58419c6e8274742c6a3ddabff6b4cdd9

stackforge/anvil,stackforge/anvil,mc2014/anvil,mc2014/anvil