internal/copyright: skip testdata and vendor directories only

Using filepath.SkipDir without confirming that d is a directory makes
it prone to taking unintended action if a file (not a directory) with
the same name gets added.

This isn't a problem today, but we shouldn't spend human code review
time checking that this doesn't somehow happen in the future, either.

Change-Id: I29bf203ddef175c3ad23c9ddc10fa934126ac853
Reviewed-on: https://go-review.googlesource.com/c/go/+/630635
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>

Author: dmitshur

src/internal/copyright/copyright_test.go

@@ -34,7 +34,7 @@ var permitted = [][]byte{
 func TestCopyright(t *testing.T) {
 	buf := make([]byte, 2048)
 	filepath.WalkDir(filepath.Join(testenv.GOROOT(t), "src"), func(path string, d fs.DirEntry, err error) error {
-		if d.Name() == "testdata" || d.Name() == "vendor" {
+		if d.IsDir() && (d.Name() == "testdata" || d.Name() == "vendor") {
 			return filepath.SkipDir
 		}
 		switch filepath.Ext(d.Name()) {