Fix file hash caching for apt backend.

[JC3]

Cache file hashes across freight cache runs for apt backend. It seems to work. Attempt to address #123.

The infrastructure for this already existed, it's just that it stored the cache files in the always-freshly-created work directory, so the previous run's cache data never carried over. This essentially just moves those files into $VARLIB instead of $DISTCACHE.

For a 23GB registry, freight cache times are decreased from 9m 30s to about 7 seconds.

TODO:

• Remove some temporary debug output.
• Fix: Remove cached hashes only for $DIST.
• Implement $CACHE != "on".
• Address failed CI checks - they seem to have been pre-existing.
• Further testing (e.g. by modifying existing packages, verifying hashes are correct, etc)

Implementation notes:

• Stores cached hashes in /var/lib/freight/.apt/ (or wherever $VARLIB is).
• Can't store next to cached *-control files because info about where those are is unavailable at the point of hash generation.
• They will be removed by freight-clear-cache.
• Cached data will only be used if $CACHE is "on".