Pin cache entries still in use

[epage]

Problem

Regardless of what automatic cleanup schedules we or the user chooses, we can end up removing files people need and they might not find out until they run in offline mode.

Proposed Solution

With #13136, we should know where all lockfiles are. We at least could pin all locally non-recreatable cache entries (e.g. index entries, .crate files) so that they don't get cleaned up, regardless of their age. Whether we should also pin recreatable cache entries is less clear though . It could help when a projects blow past low limits (see #12634 (comment)).

The main risk is the performance hit of reading all lockfiles and matching those to cache entries.

Notes

Blocked on #13136