internal/restic: Use IDSet.Clone + use maps package

One place where IDSet.Clone is useful was reinventing it, using a
conversion to list, a sort, and a conversion back to map.

Also, use the stdlib "maps" package to implement as much of IDSet as
possible. This requires changing one caller, which assumed that cloning
nil would return a non-nil IDSet.

internal/repository/index/master_index.go

@@ -347,6 +347,9 @@ func (mi *MasterIndex) Rewrite(ctx context.Context, repo restic.Unpacked, exclud
 
 	// copy excludePacks to prevent unintended sideeffects
 	excludePacks = excludePacks.Clone()
+	if excludePacks == nil {
+		excludePacks = restic.NewIDSet()
+	}
 	debug.Log("start rebuilding index of %d indexes, excludePacks: %v", len(indexes), excludePacks)
 	wg, wgCtx := errgroup.WithContext(ctx)