This is a tool to tell us which of our on-disk wheels are duplicated
upstream by PyPI. These are things we don't need to cache locally.
At one time, we were downloading all dependencies of our requirements
and caching them; we shouldn't be doing that any more, but anything
reported by this tool can be removed from our local mirrors.
Now that the number of platforms * number of branches is becoming a
maintence issue, this will help us foucs on keeping a useful working
set in the cache.
Change-Id: I3ded6b9869598a0907d7cda9f03bf414e46885df
