Enhancement: `find --pack` fallback to index if data file is missing

When investigating a repository with missing data files, it might be useful
to determine affected snapshots before running `rebuild-index`.
`restic find --pack pack-id` previously returned no data as it required
accessing the data file. Now, if the necessary data is still available in the
repository index, it gets retrieved from there.

The command now also supports looking up multiple pack files in a single `find`
run.

https://github.com/restic/restic/pull/3427
https://forum.restic.net/t/missing-packs-not-found/2600