A better cache clean-up and expiration policy

edit by @zivkan: a first step has been implemented, allowing package "last used in restore" date to be updated: https://github.com/NuGet/NuGet.Client/pull/4222

edit by NuGet Client Triage:

• Restore driven - A minimally impactful approach might be to run a "gc" periodically, like say once a week or once a month.
• Additional command - Community projects like NuGet Cleaner combine with updatePackageLastAccessTime

original post below:

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The user local NuGet cache has a tendency to grow unconditionally. Which results in funny tweets like this from @davidfowl:

David Fowler‏ @davidfowl 9m How big is your nuget cache? Mine is 14 gigs #nuget

Can we track when an item was last hit by package restore and apply some expiration policy? I don't have good data, but I bet a good chunk of the bloat aren't individual packages with a single version but a small number of packages with many versions. If were to just delete older versions that weren't needed for restore for a while, we're probably getting to something saner.

Thoughts?