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/mtimefs_test.go

@@ -25,7 +25,7 @@ func TestMtimeFS(t *testing.T) {
 	// a random time with nanosecond precision
 	testTime := time.Unix(1234567890, 123456789)
 
-	mtimefs := NewMtimeFS(make(mapStore))
+	mtimefs := NewMtimeFS(DefaultFilesystem, make(mapStore))
 
 	// Do one Chtimes call that will go through to the normal filesystem
 	osChtimes = os.Chtimes