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/fs/lstat_regular.go

@@ -0,0 +1,15 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build !linux,!android
+
+package fs
+
+import "os"
+
+func underlyingLstat(name string) (fi os.FileInfo, err error) {
+	return os.Lstat(name)
+}