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>

modelmigration/v1_20/v256.go

@@ -0,0 +1,21 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_20
+
+import "gitea.dev/modelmigration/base"
+
+func AddIsInternalColumnToPackage(x base.EngineMigration) error {
+	type Package struct {
+		ID               int64  `xorm:"pk autoincr"`
+		OwnerID          int64  `xorm:"UNIQUE(s) INDEX NOT NULL"`
+		RepoID           int64  `xorm:"INDEX"`
+		Type             string `xorm:"UNIQUE(s) INDEX NOT NULL"`
+		Name             string `xorm:"NOT NULL"`
+		LowerName        string `xorm:"UNIQUE(s) INDEX NOT NULL"`
+		SemverCompatible bool   `xorm:"NOT NULL DEFAULT false"`
+		IsInternal       bool   `xorm:"NOT NULL DEFAULT false"`
+	}
+
+	return x.Sync(new(Package))
+}