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.

internal/restorer/filerestorer.go

@@ -7,7 +7,6 @@ import (
 
 	"golang.org/x/sync/errgroup"
 
-	"github.com/restic/restic/internal/crypto"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/repository"
@@ -45,11 +44,12 @@ type packInfo struct {
 	files map[*fileInfo]struct{} // set of files that use blobs from this pack
 }
 
+type blobsLoaderFn func(ctx context.Context, packID restic.ID, blobs []restic.Blob, handleBlobFn func(blob restic.BlobHandle, buf []byte, err error) error) error
+
 // fileRestorer restores set of files
 type fileRestorer struct {
-	key        *crypto.Key
-	idx        func(restic.BlobHandle) []restic.PackedBlob
-	packLoader repository.BackendLoadFn
+	idx         func(restic.BlobHandle) []restic.PackedBlob
+	blobsLoader blobsLoaderFn
 
 	workerCount int
 	filesWriter *filesWriter
@@ -63,8 +63,7 @@ type fileRestorer struct {
 }
 
 func newFileRestorer(dst string,
-	packLoader repository.BackendLoadFn,
-	key *crypto.Key,
+	blobsLoader blobsLoaderFn,
 	idx func(restic.BlobHandle) []restic.PackedBlob,
 	connections uint,
 	sparse bool,
@@ -74,9 +73,8 @@ func newFileRestorer(dst string,
 	workerCount := int(connections)
 
 	return &fileRestorer{
-		key:         key,
 		idx:         idx,
-		packLoader:  packLoader,
+		blobsLoader: blobsLoader,
 		filesWriter: newFilesWriter(workerCount),
 		zeroChunk:   repository.ZeroChunk(),
 		sparse:      sparse,
@@ -310,7 +308,7 @@ func (r *fileRestorer) downloadBlobs(ctx context.Context, packID restic.ID,
 	for _, entry := range blobs {
 		blobList = append(blobList, entry.blob)
 	}
-	return repository.StreamPack(ctx, r.packLoader, r.key, packID, blobList,
+	return r.blobsLoader(ctx, packID, blobList,
 		func(h restic.BlobHandle, blobData []byte, err error) error {
 			processedBlobs.Insert(h)
 			blob := blobs[h.ID]