Support pex3 cache operation at the distribution level.

[jsirois]

It would be useful to be able to manipulate distributions in the Pex cache since these are the chief contributors to cache size and the cache items most directly identifiable to an end user since they ask for them in requirements.

Subject to spelling, the actions would be:

• pex3 cache add a distribution to prep for offline use.
• pex3 cache remove one or more distributions safely (paying mind to unzipped_pexes and venvs symlinks).
• pex3 cache list to list distributions and their sizes much like the broader pex3 cache info.