Package promotion with multi-user caches

[groutr]

This is an idea for a tool to complement the new multi-user caches in conda 4.3+.

Idea: If a package exists in many user's personal caches, then the package can be promoted to the global cache and re-linked from there. This would deduplicate the package from the various user caches, thereby potentially saving a lot of disk space. One strategy would be to promote a package only if more than x% of the users have it linked into their environments.

Such a tool would require knowledge about the location of all the various user caches and environments. It needs to be able to determine what exact packages are installed in each user environment. It should preserve link type (though I think it only really makes sense to promote hardlinked packages). The tool should also be mindful of potential dev installs, or user edits to files in the package cache and not count those packages for promotion.

A first step would be to simply produce a list of packages that can be promoted.