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/rwfolder_test.go

@@ -84,7 +84,7 @@ func setUpSendReceiveFolder(model *Model) sendReceiveFolder {
 			model:        model,
 		},
 
-		mtimeFS:   fs.NewMtimeFS(db.NewNamespacedKV(model.db, "mtime")),
+		mtimeFS:   fs.NewMtimeFS(fs.DefaultFilesystem, db.NewNamespacedKV(model.db, "mtime")),
 		dir:       "testdata",
 		queue:     newJobQueue(),
 		errors:    make(map[string]string),
@@ -238,7 +238,7 @@ func TestCopierFinder(t *testing.T) {
 	}
 
 	// Verify that the fetched blocks have actually been written to the temp file
-	blks, err := scanner.HashFile(tempFile, protocol.BlockSize, nil, false)
+	blks, err := scanner.HashFile(fs.DefaultFilesystem, tempFile, protocol.BlockSize, nil, false)
 	if err != nil {
 		t.Log(err)
 	}