refactor: clean up git repo and model migration packages (#38564)

enable the golangci depguard lint rule: deny "models" and its sub
packages in "modelmigration" package.

modelmigration/base/shared.go

@@ -0,0 +1,25 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package base
+
+import (
+	"gitea.dev/models/db" //nolint:depguard // allow to access db in migration
+	"gitea.dev/modules/git/gitrepo"
+)
+
+// HINT: MIGRATION-STRUCT-FROZEN: the structs used in migrations should not be affected by the changes happen in the future,
+// because the details 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 existing migration will break because it will use the new (incorrect) User model,
+// it should only use the old User model.
+//
+// Related: "models", "modules/structs", git repo directory layout on the disk, etc.
+//
+// If changes happen, the old migrations need to use a snapshot struct/function (copy the code and freeze)
+
+type ResourceIndex = db.ResourceIndex
+
+func LocalCodeGitRepo(ownerName, repoName string) gitrepo.RepositoryFacade {
+	return gitrepo.CodeRepoByName(ownerName, repoName)
+}