cmd/cgo: add import path to hash

This avoids name conflicts when two identical packages use cgo.
This can happen in practice when the same package is vendored multiple
times in a single build.

Fixes #23555

Change-Id: I9f0ec6db9165dcf9cdf3d314c668fee8ada18f9c
Reviewed-on: https://go-review.googlesource.com/118739
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/issue23555.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that we can have two identical cgo packages in a single binary.
+// No runtime test; just make sure it compiles.
+
+package cgotest
+
+import _ "./issue23555a"
+import _ "./issue23555b"

misc/cgo/test/issue23555a/a.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue23555
+
+// #include <stdlib.h>
+import "C"
+
+func X() {
+	C.free(C.malloc(10))
+}

misc/cgo/test/issue23555b/a.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue23555
+
+// #include <stdlib.h>
+import "C"
+
+func X() {
+	C.free(C.malloc(10))
+}

src/cmd/cgo/main.go

@@ -17,6 +17,7 @@ import (
 	"go/ast"
 	"go/printer"
 	"go/token"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -279,6 +280,7 @@ func main() {
 	// concern is other cgo wrappers for the same functions.
 	// Use the beginning of the md5 of the input to disambiguate.
 	h := md5.New()
+	io.WriteString(h, *importPath)
 	fs := make([]*File, len(goFiles))
 	for i, input := range goFiles {
 		if *srcDir != "" {