internal/mod/modfile: add copyright message

As suggested in https://cuelang.org/cl/1168707 and not done before
it landed.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: Ib412b2646a694a417c54511fcabee1703b5b7852
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171838
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

internal/mod/modfile/modfile_test.go

@@ -73,8 +73,8 @@ langugage: version: "v0.4.3"
 module: "foo.com/bar@v0"
 `,
 	wantError: `langugage: field not allowed:
-    cuelang.org/go/internal/mod/modfile/schema.cue:14:8
-    cuelang.org/go/internal/mod/modfile/schema.cue:16:2
+    cuelang.org/go/internal/mod/modfile/schema.cue:28:8
+    cuelang.org/go/internal/mod/modfile/schema.cue:30:2
     module.cue:2:1`,
 }, {
 	testName: "InvalidLanguageVersion",

internal/mod/modfile/schema.cue

@@ -1,3 +1,17 @@
+// Copyright 2023 CUE 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.
+
 // This schema constrains a module.cue file. This form constrains module.cue files
 // outside of the main module. For the module.cue file in a main module, the schema
 // is less restrictive, because wherever #Semver is used, a less specific version may be