Fix for buildout 2.2.3 upgrade/downgrade problems

Fix for #198 

I'm going home; TODO: changelog entry. Functionality works.

Note: I'm getting one test error (number of log handlers, unrelated).
Quick note: removing the `ws.by_key.get(req.key)` line is the single most important part of this pull request.

This was the line that accepted every package, either in the working set or in the system-wide path, regardless of the version :-)