Refactor find forks and fix possible bugs that weak permissions check (#32528) (#32547)

Backport #32528

- Move models/GetForks to services/FindForks
- Add doer as a parameter of FindForks to check permissions
- Slight performance optimization for get forks API with batch loading
of repository units
- Add tests for forking repository to organizations

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/repo/fork.go

@@ -54,21 +54,6 @@ func GetUserFork(ctx context.Context, repoID, userID int64) (*Repository, error)
 	return &forkedRepo, nil
 }
 
-// GetForks returns all the forks of the repository
-func GetForks(ctx context.Context, repo *Repository, listOptions db.ListOptions) ([]*Repository, error) {
-	sess := db.GetEngine(ctx)
-
-	var forks []*Repository
-	if listOptions.Page == 0 {
-		forks = make([]*Repository, 0, repo.NumForks)
-	} else {
-		forks = make([]*Repository, 0, listOptions.PageSize)
-		sess = db.SetSessionPagination(sess, &listOptions)
-	}
-
-	return forks, sess.Find(&forks, &Repository{ForkID: repo.ID})
-}
-
 // IncrementRepoForkNum increment repository fork number
 func IncrementRepoForkNum(ctx context.Context, repoID int64) error {
 	_, err := db.GetEngine(ctx).Exec("UPDATE `repository` SET num_forks=num_forks+1 WHERE id=?", repoID)