cmd/go: support new location for vet

Also delete the special case for exp, which isn't necessary any more.

Fixes #5529.

R=rsc, nightlyone
CC=golang-dev
https://golang.org/cl/9611048

src/cmd/go/pkg.go

@@ -275,8 +275,8 @@ var isGoTool = map[string]bool{
 	"cmd/api":                            true,
 	"cmd/cgo":                            true,
 	"cmd/fix":                            true,
-	"cmd/vet":  true,
 	"cmd/yacc":                           true,
+	"code.google.com/p/go.tools/cmd/vet": true,
 }
 
 // expandScanner expands a scanner.List error into all the errors in the list.
@@ -329,7 +329,7 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		if p.build.BinDir != "" {
 			p.target = filepath.Join(p.build.BinDir, elem)
 		}
-		if p.Goroot && (isGoTool[p.ImportPath] || strings.HasPrefix(p.ImportPath, "exp/")) {
+		if isGoTool[p.ImportPath] {
 			p.target = filepath.Join(gorootPkg, "tool", full)
 		}
 		if p.target != "" && buildContext.GOOS == "windows" {

src/cmd/go/tool.go

@@ -45,12 +45,30 @@ func init() {
 
 const toolWindowsExtension = ".exe"
 
-func tool(name string) string {
-	p := filepath.Join(toolDir, name)
-	if toolIsWindows && name != "pprof" {
-		p += toolWindowsExtension
+func tool(toolName string) string {
+	toolPath := filepath.Join(toolDir, toolName)
+	if toolIsWindows && toolName != "pprof" {
+		toolPath += toolWindowsExtension
 	}
-	return p
+	// Give a nice message if there is no tool with that name.
+	if _, err := os.Stat(toolPath); err != nil {
+		if isInGoToolsRepo(toolName) {
+			fmt.Fprintf(os.Stderr, "go tool: no such tool %q; to install:\n\tgo install code.google.com/p/go.tools/cmd/%s\n", toolName, toolName)
+		} else {
+			fmt.Fprintf(os.Stderr, "go tool: no such tool %q\n", toolName)
+		}
+		setExitStatus(3)
+		exit()
+	}
+	return toolPath
+}
+
+func isInGoToolsRepo(toolName string) bool {
+	switch toolName {
+	case "vet":
+		return true
+	}
+	return false
 }
 
 func runTool(cmd *Command, args []string) {
@@ -70,10 +88,7 @@ func runTool(cmd *Command, args []string) {
 		}
 	}
 	toolPath := tool(toolName)
-	// Give a nice message if there is no tool with that name.
-	if _, err := os.Stat(toolPath); err != nil {
-		fmt.Fprintf(os.Stderr, "go tool: no such tool %q\n", toolName)
-		setExitStatus(3)
+	if toolPath == "" {
 		return
 	}
 	if toolIsWindows && toolName == "pprof" {
@@ -86,7 +101,6 @@ func runTool(cmd *Command, args []string) {
 			return
 		}
 	}
-
 	if toolN {
 		fmt.Printf("%s %s\n", toolPath, strings.Join(args[1:], " "))
 		return