backends: make NewObject return fs.ErrorIsDir if possible

This changes the interface to NewObject so that if NewObject is called
on a directory then it should return fs.ErrorIsDir if possible without
doing any extra work, otherwise fs.ErrorObjectNotFound.

Tested on integration test server with:

go run integration-test.go -tests backend -run TestIntegration/FsMkdir/FsPutFiles/FsNewObjectDir -branch fix-stat -maxtries 1

backend/yandex/yandex.go

@@ -949,7 +949,9 @@ func (o *Object) readMetaData(ctx context.Context) (err error) {
 	if err != nil {
 		return err
 	}
-	if info.ResourceType != "file" {
+	if info.ResourceType == "dir" {
+		return fs.ErrorIsDir
+	} else if info.ResourceType != "file" {
 		return fs.ErrorNotAFile
 	}
 	return o.setMetaData(info)