lib/scanner: Use fs.Filesystem for all operations

One more step on the path of the great refactoring. Touches rwfolder a
little bit since it uses the Lstat from fs as well, but mostly this is
just on the scanner as rwfolder is scheduled for a later refactor.

There are a couple of usages of fs.DefaultFilesystem that will in the
end become a filesystem injected from the top, but that comes later.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4070
LGTM: AudriusButkevicius, imsodin

lib/db/set.go

@@ -303,7 +303,7 @@ func (s *FileSet) SetIndexID(device protocol.DeviceID, id protocol.IndexID) {
 func (s *FileSet) MtimeFS() *fs.MtimeFS {
 	prefix := s.db.mtimesKey([]byte(s.folder))
 	kv := NewNamespacedKV(s.db, string(prefix))
-	return fs.NewMtimeFS(kv)
+	return fs.NewMtimeFS(fs.DefaultFilesystem, kv)
 }
 
 func (s *FileSet) ListDevices() []protocol.DeviceID {