cmd/go: always add 'go' directive to the go.mod file if missing

Updates #30790
Fixes #31960

Change-Id: Ib3ac074cf1f98fe69f53af82d2a0441582116570
Reviewed-on: https://go-review.googlesource.com/c/go/+/176925
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modcmd/tidy.go

@@ -64,7 +64,6 @@ func runTidy(cmd *base.Command, args []string) {
 		}
 	}
 	modload.SetBuildList(keep)
-	modload.AddGoStmt()
 	modTidyGoSum() // updates memory copy; WriteGoMod on next line flushes it out
 	modload.WriteGoMod()
 }

src/cmd/go/internal/modload/init.go

@@ -421,7 +421,7 @@ func legacyModInit() {
 		fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", path)
 		modFile = new(modfile.File)
 		modFile.AddModuleStmt(path)
-		AddGoStmt()
+		addGoStmt() // Add the go directive before converted module requirements.
 	}
 
 	for _, name := range altConfigs {
@@ -432,7 +432,6 @@ func legacyModInit() {
 			if convert == nil {
 				return
 			}
-			AddGoStmt()
 			fmt.Fprintf(os.Stderr, "go: copying requirements from %s\n", base.ShortPath(cfg))
 			cfg = filepath.ToSlash(cfg)
 			if err := modconv.ConvertLegacyConfig(modFile, cfg, data); err != nil {
@@ -447,9 +446,9 @@ func legacyModInit() {
 	}
 }
 
-// AddGoStmt adds a go directive to the go.mod file if it does not already include one.
+// addGoStmt adds a go directive to the go.mod file if it does not already include one.
 // The 'go' version added, if any, is the latest version supported by this toolchain.
-func AddGoStmt() {
+func addGoStmt() {
 	if modFile.Go != nil && modFile.Go.Version != "" {
 		return
 	}
@@ -653,6 +652,8 @@ func WriteGoMod() {
 		return
 	}
 
+	addGoStmt()
+
 	if loaded != nil {
 		reqs := MinReqs()
 		min, err := reqs.Required(Target)

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

@@ -21,6 +21,7 @@ grep v1.5.1 $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/list
 
 -- $WORK/x/go.mod --
 module x
+go 1.13
 require rsc.io/quote v1.5.1
 -- $WORK/x/x.go --
 package x

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

@@ -10,6 +10,8 @@ stdout '^example.com$'
 
 -- go.mod --
 module example.com
+
+go 1.13
 -- main.go --
 package main
 

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

@@ -37,6 +37,8 @@ cmp stderr list-missing-test-expected
 -- go.mod.orig --
 module m
 
+go 1.13
+
 require example.com/badchain/a v1.0.0
 -- use/use.go --
 package use