refactor: prepare to decouple the "model migration" package and "models" package (#38533)

Migrations should never use model structs directly, because the model
structs can be different in different releases. e.g. if one migration uses
"User" model, it works in the early releases, then one day, when the
User model changes, the migration breaks because it will use the
new (incorrect) User model, it should only use the old User model.

The same to "modules/structs".

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

routers/common/db.go

@@ -8,8 +8,8 @@ import (
 	"errors"
 	"time"
 
+	"gitea.dev/modelmigration"
 	"gitea.dev/models/db"
-	"gitea.dev/models/migrations"
 	system_model "gitea.dev/models/system"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
@@ -45,12 +45,12 @@ func migrateWithSetting(ctx context.Context, x db.EngineMigration) error {
 		return versioned_migration.Migrate(ctx, x)
 	}
 
-	if current, err := migrations.GetCurrentDBVersion(x); err != nil {
+	if current, err := modelmigration.GetCurrentDBVersion(x); err != nil {
 		return err
 	} else if current < 0 {
 		// execute migrations when the database isn't initialized even if AutoMigration is false
 		return versioned_migration.Migrate(ctx, x)
-	} else if expected := migrations.ExpectedDBVersion(); current != expected {
+	} else if expected := modelmigration.ExpectedDBVersion(); current != expected {
 		log.Fatal(`"database.AUTO_MIGRATION" is disabled, but current database version %d is not equal to the expected version %d.`+
 			`You can set "database.AUTO_MIGRATION" to true or migrate manually by running "gitea [--config /path/to/app.ini] migrate"`, current, expected)
 	}