cmd/go: mention internal package path in 'use of internal package not allowed' error

Fixes golang/go#25164.

Change-Id: Ieca2a22b797ada69a768cb124d03bc74517e4198
Reviewed-on: https://go-review.googlesource.com/116761
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: rsc

vendor/cmd/go/go_test.go

@@ -1248,14 +1248,14 @@ func TestInternalPackagesInGOROOTAreRespected(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.runFail("build", "-v", "./testdata/testinternal")
-	tg.grepBoth(`testinternal(\/|\\)p\.go\:3\:8\: use of internal package not allowed`, "wrong error message for testdata/testinternal")
+	tg.grepBoth(`testinternal(\/|\\)p\.go\:3\:8\: use of internal package net/http/internal not allowed`, "wrong error message for testdata/testinternal")
 }
 
 func TestInternalPackagesOutsideGOROOTAreRespected(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.runFail("build", "-v", "./testdata/testinternal2")
-	tg.grepBoth(`testinternal2(\/|\\)p\.go\:3\:8\: use of internal package not allowed`, "wrote error message for testdata/testinternal2")
+	tg.grepBoth(`testinternal2(\/|\\)p\.go\:3\:8\: use of internal package .*internal/w not allowed`, "wrote error message for testdata/testinternal2")
 }
 
 func TestRunInternal(t *testing.T) {
@@ -1265,7 +1265,7 @@ func TestRunInternal(t *testing.T) {
 	tg.setenv("GOPATH", dir)
 	tg.run("run", filepath.Join(dir, "src/run/good.go"))
 	tg.runFail("run", filepath.Join(dir, "src/run/bad.go"))
-	tg.grepStderr(`testdata(\/|\\)src(\/|\\)run(\/|\\)bad\.go\:3\:8\: use of internal package not allowed`, "unexpected error for run/bad.go")
+	tg.grepStderr(`testdata(\/|\\)src(\/|\\)run(\/|\\)bad\.go\:3\:8\: use of internal package run/subdir/internal/private not allowed`, "unexpected error for run/bad.go")
 }
 
 func testMove(t *testing.T, vcs, url, base, config string) {

vendor/cmd/go/internal/load/pkg.go

@@ -730,7 +730,7 @@ func disallowInternal(srcDir string, p *Package, stk *ImportStack) *Package {
 	perr := *p
 	perr.Error = &PackageError{
 		ImportStack: stk.Copy(),
-		Err:         "use of internal package not allowed",
+		Err:         "use of internal package " + p.ImportPath + " not allowed",
 	}
 	perr.Incomplete = true
 	return &perr