repository: move Blob, Blobs and PackedBlob to pack package

This removes them from the public interface. The latter now only
provides the PackBlob interface, without being bound to the type used
internally by the pack package.

internal/repository/pack/pack_internal_test.go

@@ -182,7 +182,7 @@ func TestReadRecords(t *testing.T) {
 func TestUnpackedVerification(t *testing.T) {
 	// create random keys
 	k := crypto.NewRandomKey()
-	blobs := restic.Blobs{
+	blobs := []Blob{
 		{
 			BlobHandle:         restic.NewRandomBlobHandle(),
 			Length:             42,