Use "pack file" instead of "data file" (#2885)

- changed variable names, especially changed DataFile into PackFile
- changed in some comments
- always use "pack file" in docu

internal/backend/rclone/internal_test.go

@@ -32,7 +32,7 @@ func TestRcloneExit(t *testing.T) {
 	for i := 0; i < 10; i++ {
 		_, err = be.Stat(context.TODO(), restic.Handle{
 			Name: "foo",
-			Type: restic.DataFile,
+			Type: restic.PackFile,
 		})
 		rtest.Assert(t, err != nil, "expected an error")
 	}