wire: make Vendor test work with Go module (google/go-cloud#469)

internal/wire/testdata/Vendor/bar/dummy.go

@@ -1,16 +0,0 @@
-// Copyright 2018 The Go Cloud 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
-//
-//     https://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 bar is left intentionally blank.
-package bar