Move repository model into models/repo (#17933)

* Some refactors related repository model

* Move more methods out of repository

* Move repository into models/repo

* Fix test

* Fix test

* some improvements

* Remove unnecessary function

modules/indexer/issues/indexer_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 	"time"
 
+	_ "code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"