cmd/go/internal/imports: resolve symlinks in ScanDir

We were using the mode reported by ReadDir to decide whether each
entry is a file, but in the case of symlinks that isn't sufficient: a
symlink could point to either a file or a directory, and if it is a
file we should treat it as such.

Fixes #28107

Change-Id: Icf6e495dce427a7b1124c9cc9f085e40a215c169
Reviewed-on: https://go-review.googlesource.com/c/141097
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/imports/scan.go

@@ -22,6 +22,16 @@ func ScanDir(dir string, tags map[string]bool) ([]string, []string, error) {
 	var files []string
 	for _, info := range infos {
 		name := info.Name()
+
+		// If the directory entry is a symlink, stat it to obtain the info for the
+		// link target instead of the link itself.
+		if info.Mode()&os.ModeSymlink != 0 {
+			info, err = os.Stat(name)
+			if err != nil {
+				continue // Ignore broken symlinks.
+			}
+		}
+
 		if info.Mode().IsRegular() && !strings.HasPrefix(name, "_") && strings.HasSuffix(name, ".go") && MatchFile(name, tags) {
 			files = append(files, filepath.Join(dir, name))
 		}

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

@@ -0,0 +1,23 @@
+env GO111MODULE=on
+[!symlink] skip
+
+# 'go list' should resolve modules of imported packages.
+go list -deps -f '{{.Module}}'
+stdout golang.org/x/text
+
+# They should continue to resolve if the importing file is a symlink.
+mkdir links
+cd links
+symlink go.mod -> ../go.mod
+symlink issue.go -> ../issue.go
+
+go list -deps -f '{{.Module}}'
+stdout golang.org/x/text
+
+-- go.mod --
+module golang.org/issue/28107
+
+-- issue.go --
+package issue
+
+import _ "golang.org/x/text/language"