init-cache should reuse existing cached packages where possible

[DemiMarie]

How to file a helpful issue

The problem you're addressing (if any)

init-cache wastefully redownloads packages that have already been downloaded.

The solution you'd like

init-cache should use packages that have already been downloaded where possible. If this is combined with #9703, one can go even further and do an in-place upgrade of the chroot.

The value to a user, and who that user might be

All contributors will benefit from faster init-cache runs.

Completion criteria checklist

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