restic: change ListBlobs to return PackBlob

PackBlob is a limited interface that only exposes a part of the
information provided by PackedBlob. Most of the changes are switches
from direct value lookups to the interface methods, with a few larger
changes to let the tests still work.

internal/repository/index_testutil_test.go

@@ -0,0 +1,17 @@
+package repository
+
+import (
+	"github.com/restic/restic/internal/restic"
+)
+
+// BlobsInPack returns index entries for blobs stored in packID, sorted by offset.
+func BlobsInPack(repo *Repository, packID restic.ID) restic.Blobs {
+	var blobs restic.Blobs
+	for pb := range repo.idx.Values() {
+		if pb.PackID.Equal(packID) {
+			blobs = append(blobs, pb.Blob)
+		}
+	}
+	blobs.Sort()
+	return blobs
+}