cmd/go/internal/modload: skip go.mod directories on all systems, not just Plan 9

I see no reason Plan 9 should be special cased.
A directory named go.mod is not useful on any system.
Followup to CL 129804.

Change-Id: I9cc91b5934b17650bfdb07370aa73aeae445968c
Reviewed-on: https://go-review.googlesource.com/c/149337
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -25,7 +25,6 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strconv"
 	"strings"
 )
@@ -402,7 +401,7 @@ func FindModuleRoot(dir, limit string, legacyConfigOK bool) (root, file string)
 
 	// Look for enclosing go.mod.
 	for {
-		if fi, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil && !(runtime.GOOS == "plan9" && fi.IsDir()) {
+		if fi, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil && !fi.IsDir() {
 			return dir, "go.mod"
 		}
 		if dir == limit {
@@ -420,7 +419,7 @@ func FindModuleRoot(dir, limit string, legacyConfigOK bool) (root, file string)
 		dir = dir1
 		for {
 			for _, name := range altConfigs {
-				if fi, err := os.Stat(filepath.Join(dir, name)); err == nil && !(runtime.GOOS == "plan9" && fi.IsDir()) {
+				if fi, err := os.Stat(filepath.Join(dir, name)); err == nil && !fi.IsDir() {
 					return dir, name
 				}
 			}