repository: Replace StreamPack function with LoadBlobsFromPack method

LoadBlobsFromPack is now part of the repository struct. This ensures
that users of that method don't have to deal will internals of the
repository implementation.

The filerestorer tests now also contain far fewer pack file
implementation details.

cmd/restic/cmd_repair_packs.go

@@ -116,7 +116,7 @@ func repairPacks(ctx context.Context, gopts GlobalOptions, repo *repository.Repo
 				continue
 			}
 
-			err = repository.StreamPack(wgCtx, repo.Backend().Load, repo.Key(), b.PackID, blobs, func(blob restic.BlobHandle, buf []byte, err error) error {
+			err = repo.LoadBlobsFromPack(wgCtx, b.PackID, blobs, func(blob restic.BlobHandle, buf []byte, err error) error {
 				if err != nil {
 					// Fallback path
 					buf, err = repo.LoadBlob(wgCtx, blob.Type, blob.ID, nil)