cmd/internal/moddeps: don't skip directories if there are unusual files

Previously, if there was a non-directory file with the name vendor or
testdata in the Go source tree, it was possible for some directories
to be skipped by filepath.Walk performed in findGorootModules.

As unusual and unlikely as such non-directory files are, it's better
to ensure all directories are visited, and all modules in the GOROOT
source tree are found.

This increases confidence that tests relying on findGorootModule
will not have unexpected false negatives.

For #36851.
For #36907.

Change-Id: I468e80d8f57119e2c72d546b3fd1e23c31fd6e6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/236600
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: dmitshur

src/cmd/internal/moddeps/moddeps_test.go

@@ -36,7 +36,7 @@ func findGorootModules(t *testing.T) []gorootModule {
 			if err != nil {
 				return err
 			}
-			if info.Name() == "vendor" || info.Name() == "testdata" {
+			if info.IsDir() && (info.Name() == "vendor" || info.Name() == "testdata") {
 				return filepath.SkipDir
 			}
 			if path == filepath.Join(runtime.GOROOT(), "pkg") {