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`. Previously,
`find --pack pack-id` 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
