tpl: Make {{ template "partials/foo.html" . }} work in older setups

Fixes #13599

tpl/tplimpl/templatestore.go

@@ -1,3 +1,18 @@
+// Copyright 2025 The Hugo Authors. All rights reserved.
+//
+// Portions Copyright The Go Authors.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package tplimpl
 
 import (