cmd/go: display correct options in "go help get" using modules

Fixes: #27298

Change-Id: Icfc6992b470136bb25a77912f670a25883642316
Reviewed-on: https://go-review.googlesource.com/132095
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/main.go

@@ -93,6 +93,15 @@ func main() {
 		*get.CmdGet = *modget.CmdGet
 	}
 
+	if args[0] == "get" || args[0] == "help" {
+		// Replace get with module-aware get if appropriate.
+		// Note that if MustUseModules is true, this happened already above,
+		// but no harm in doing it again.
+		if modload.Init(); modload.Enabled() {
+			*get.CmdGet = *modget.CmdGet
+		}
+	}
+
 	cfg.CmdName = args[0] // for error messages
 	if args[0] == "help" {
 		help.Help(os.Stdout, args[1:])
@@ -161,15 +170,6 @@ func main() {
 		os.Exit(2)
 	}
 
-	if args[0] == "get" {
-		// Replace get with module-aware get if appropriate.
-		// Note that if MustUseModules is true, this happened already above,
-		// but no harm in doing it again.
-		if modload.Init(); modload.Enabled() {
-			*get.CmdGet = *modget.CmdGet
-		}
-	}
-
 	// Set environment (GOOS, GOARCH, etc) explicitly.
 	// In theory all the commands we invoke should have
 	// the same default computation of these as we do,

src/cmd/go/testdata/script/help.txt

@@ -34,3 +34,8 @@ stderr 'Run ''go help mod'' for usage.'
 ! go vet -h
 stderr 'usage: go vet'
 stderr 'Run ''go help vet'' for details'
+
+# go help get shows usage for get
+go help get
+stdout 'usage: go get'
+stdout 'get when using GOPATH'

src/cmd/go/testdata/script/mod_help.txt

@@ -0,0 +1,6 @@
+env GO111MODULE=on
+
+# go help get shows usage for get
+go help get
+stdout 'usage: go get'
+stdout 'get using modules to manage source'