cmd: ignore the directory named go.mod

The existing implementation does not check in all cases whether go.mod is a regular file.

Fixes #30788

Change-Id: I6d140545c3cfada651612efd5bee2fbdcb747ca7
GitHub-Last-Rev: 4a9b251e378d9d7cc8768d395c360d3542fc9bc6
GitHub-Pull-Request: golang/go#30830
Reviewed-on: https://go-review.googlesource.com/c/go/+/167393
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/import.go

@@ -233,8 +233,8 @@ func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFile
 	if isLocal {
 		for d := dir; d != mdir && len(d) > len(mdir); {
 			haveGoMod := haveGoModCache.Do(d, func() interface{} {
-				_, err := os.Stat(filepath.Join(d, "go.mod"))
-				return err == nil
+				fi, err := os.Stat(filepath.Join(d, "go.mod"))
+				return err == nil && !fi.IsDir()
 			}).(bool)
 
 			if haveGoMod {

src/cmd/go/internal/modload/search.go

@@ -76,7 +76,7 @@ func matchPackages(pattern string, tags map[string]bool, useStd bool, modules []
 			}
 			// Stop at module boundaries.
 			if path != root {
-				if _, err := os.Stat(filepath.Join(path, "go.mod")); err == nil {
+				if fi, err := os.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
 					return filepath.SkipDir
 				}
 			}

src/cmd/go/internal/search/search.go

@@ -190,7 +190,7 @@ func MatchPackagesInFS(pattern string) *Match {
 
 		if !top && cfg.ModulesEnabled {
 			// Ignore other modules found in subdirectories.
-			if _, err := os.Stat(filepath.Join(path, "go.mod")); err == nil {
+			if fi, err := os.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
 				return filepath.SkipDir
 			}
 		}

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

+# The directory named go.mod should be ignored
+
+env GO111MODULE=on
+
+cd $WORK/sub
+
+go list .
+stdout 'x/sub'
+
+mkdir go.mod
+exists go.mod
+
+go list .
+stdout 'x/sub'
+
+-- $WORK/go.mod --
+module x
+
+-- $WORK/sub/x.go --
+package x
\ No newline at end of file