Enhancement: Check now checks index when reading packs

Restic used to only verfiy the pack file content when calling `check --read-data`
`check --read-data-subset` but did not check if the blobs within the pack are
correctly contained in the index.
This check is now added and may give an "Blob ID is not contained in index" error.
If the index is not correct, it can be rebuilt by using the `rebuild-index` command.

https://github.com/restic/restic/pull/3048