goose: allow non-injector code to live along with injectors

Previously, goose would ignore declarations in the //+build gooseinject
files that were not injectors. This meant that if you wanted to write
application-specific providers, you would need to place them in a
separate file, away from the goose injectors. This means that a typical
application would have three handwritten files: one for the abstract
business logic, one for the platform-specific providers, one for the
platform-specific injector declarations.

This change allows the two platform-specific files to be merged into
one: the //+build gooseinject file. goose will now copy these
declarations out to goose_gen.go. This requires a bit of hackery, since
the generated file may have different identifiers for the imported
packages, so goose will do some light AST rewriting to address these
cases.

(Historical note: this was the first change made externally, so also in
here are the copyright headers and other housekeeping changes.)

Reviewed-by: Tuo Shan <shantuo@google.com>
Reviewed-by: kokoro <noreply+kokoro@google.com>

internal/goose/testdata/NamingWorstCaseAllInOne/foo/foo.go

@@ -0,0 +1,58 @@
+// 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 gooseinject
+
+// This file is specifically designed to cause issues with copying the
+// AST, particularly with the identifier "context".
+
+package main
+
+import (
+	stdcontext "context"
+	"fmt"
+	"os"
+	"reflect"
+
+	"github.com/google/go-cloud/goose"
+)
+
+type context struct{}
+
+func main() {
+	if _, ok := reflect.TypeOf(context{}).MethodByName("Provide"); !ok {
+		fmt.Println("ERROR: context.Provide renamed")
+		os.Exit(1)
+	}
+	c, err := inject(stdcontext.Background(), struct{}{})
+	if err != nil {
+		fmt.Println("ERROR:", err)
+		os.Exit(1)
+	}
+	fmt.Println(c)
+}
+
+func Provide(context2 stdcontext.Context) (context, error) {
+	var context3 = stdcontext.Background()
+	_ = context2
+	_ = context3
+	return context{}, nil
+}
+
+func inject(context stdcontext.Context, err struct{}) (context, error) {
+	panic(goose.Use(Provide))
+}
+
+func (context) Provide() {
+}

internal/goose/testdata/NamingWorstCaseAllInOne/out.txt

@@ -0,0 +1 @@
+{}

internal/goose/testdata/NamingWorstCaseAllInOne/pkg

@@ -0,0 +1 @@
+foo