[cipd] Add AssertOnlyDefaultSubdir.

cipd/client/cmd/cipd/main.go

Index: cipd/client/cmd/cipd/main.go
diff --git a/cipd/client/cmd/cipd/main.go b/cipd/client/cmd/cipd/main.go
index 54b973d0b5670747ecb385fbfd805feaacc202f0..61e1d674a0c9cb168d1da73e32b4248f75e16ee0 100644
--- a/cipd/client/cmd/cipd/main.go
+++ b/cipd/client/cmd/cipd/main.go
@@ -767,11 +767,10 @@ func ensurePackages(ctx context.Context, root string, desiredStateFile string, d
 		return nil, cipd.Actions{}, err
 	}
 
-	baseSubdir := resolved.PackagesBySubdir[""]
-	delete(resolved.PackagesBySubdir, "")
-	if len(resolved.PackagesBySubdir) > 0 {
-		return nil, cipd.Actions{}, errors.New("@Subdir not yet supported")
+	if err := resolved.PackagesBySubdir.AssertOnlyDefaultSubdir(); err != nil {
+		return nil, cipd.Actions{}, err
 	}
+	baseSubdir := resolved.PackagesBySubdir[""]
 
 	actions, err := client.EnsurePackages(ctx, baseSubdir, dryRun)
 	if err != nil {