Move convert package to services (#22264)

Addition to #22256

The `convert` package relies heavily on different models which is
[disallowed by our definition of
modules](https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md#design-guideline).
This helps to prevent possible import cycles.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

tests/integration/api_team_user_test.go

@@ -10,8 +10,8 @@ import (
 
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
-	"code.gitea.io/gitea/modules/convert"
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/services/convert"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/stretchr/testify/assert"