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/model/model.go

@@ -1820,7 +1820,7 @@ func (m *Model) internalScanFolderSubdirs(folder string, subDirs []string) error
 		BlockSize:             protocol.BlockSize,
 		TempLifetime:          time.Duration(m.cfg.Options().KeepTemporariesH) * time.Hour,
 		CurrentFiler:          cFiler{m, folder},
-		Lstater:               mtimefs,
+		Filesystem:            mtimefs,
 		IgnorePerms:           folderCfg.IgnorePerms,
 		AutoNormalize:         folderCfg.AutoNormalize,
 		Hashers:               m.numHashers(folder),