lib/scanner: Support walking a symlink root (ref #4353)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4666
LGTM: AudriusButkevicius, imsodin

lib/fs/walkfs.go

@@ -46,7 +46,7 @@ func (f *walkFilesystem) walk(path string, info FileInfo, walkFn WalkFunc) error
 		return err
 	}
 
-	if !info.IsDir() {
+	if !info.IsDir() && path != "." {
 		return nil
 	}