cmd/dist: print error if GOROOT is inside a module

Fixes #36701

Change-Id: I22738235e7a7ee06bc5d748213aab523aad8cf12
Reviewed-on: https://go-review.googlesource.com/c/go/+/215939
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Marwan Sulaiman <marwan.sameer@gmail.com>

src/cmd/dist/build.go

@@ -110,6 +110,9 @@ func xinit() {
 		fatalf("$GOROOT must be set")
 	}
 	goroot = filepath.Clean(b)
+	if modRoot := findModuleRoot(goroot); modRoot != "" {
+		fatalf("found go.mod file in %s: $GOROOT must not be inside a module", modRoot)
+	}
 
 	b = os.Getenv("GOROOT_FINAL")
 	if b == "" {
@@ -1590,6 +1593,20 @@ func checkCC() {
 	}
 }
 
+func findModuleRoot(dir string) (root string) {
+	for {
+		if fi, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil && !fi.IsDir() {
+			return dir
+		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		dir = d
+	}
+	return ""
+}
+
 func defaulttarg() string {
 	// xgetwd might return a path with symlinks fully resolved, and if
 	// there happens to be symlinks in goroot, then the hasprefix test