wire: handle build tags with relative paths correctly (google/go-cloud#188)

5cfd0141dd95acd1a98a1b7f6de5b8b304077634 worked correctly with
absolute import paths, but would fail with relative import paths
(most commonly when passing "." in the default invocation of
gowire). This commit introduces test infrastructure to allow
relative import paths to be given as test inputs, along with a test
case that addresses the regression.

internal/wire/testdata/BuildTagsRelativePkg/bar/bar.go

@@ -0,0 +1,23 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+//+build !wireinject
+
+// Package bar includes both wireinject and non-wireinject variants.
+package bar
+
+import "github.com/google/go-cloud/wire"
+
+// Set provides a friendly user greeting.
+var Set = wire.NewSet(wire.Value("Hello, World!"))

internal/wire/testdata/BuildTagsRelativePkg/bar/bar_inject.go

@@ -0,0 +1,22 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+//+build wireinject
+
+package bar
+
+import "github.com/google/go-cloud/wire"
+
+// Set provides an unfriendly user greeting.
+var Set = wire.NewSet(wire.Value("Bah humbug! This is the wrong variant!"))

internal/wire/testdata/BuildTagsRelativePkg/foo/foo.go

@@ -0,0 +1,21 @@
+// Copyright 2018 Google LLC
+//
+// 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 main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}

internal/wire/testdata/BuildTagsRelativePkg/foo/wire.go

@@ -0,0 +1,28 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+//+build wireinject
+
+package main
+
+import (
+	"bar"
+
+	"github.com/google/go-cloud/wire"
+)
+
+func injectedMessage() string {
+	wire.Build(bar.Set)
+	return ""
+}

internal/wire/testdata/BuildTagsRelativePkg/out.txt

@@ -0,0 +1 @@
+Hello, World!

internal/wire/testdata/BuildTagsRelativePkg/pkg

@@ -0,0 +1 @@
+./foo