Store content hashes in cached artifacts and use it while loading from cache

[wolfs]

Instead of rebuilding the tree hashes/file hashes when loading from the cache, we should store the hashes in the cache artifact and use them when loading from the cache instead of re-hashing the files.

This also means that we would be able to reuse the data while unpacking cached artifacts.

[lptr]

From @britter on how it is now solved:

The entry point to this is FingerprintCache and it is used in CachingRegularFileHasher and CachingResourceHasher.

[stale]

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