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/fixtures/Test_FixCommitStatusTargetURLToUseRunAndJobID/action_run.yml

@@ -0,0 +1,29 @@
+# type ActionRun struct {
+#   ID           int64 `xorm:"pk autoincr"`
+#   RepoID       int64 `xorm:"index"`
+#   Index        int64
+#   CommitSHA    string `xorm:"commit_sha"`
+#   Event        string
+#   TriggerEvent string
+#   EventPayload string `xorm:"LONGTEXT"`
+# }
+-
+  id: 990
+  repo_id: 100
+  index: 7
+  commit_sha: merge-sha
+  event: pull_request
+  event_payload: '{"pull_request":{"head":{"sha":"sha-shared"}}}'
+-
+  id: 991
+  repo_id: 100
+  index: 8
+  commit_sha: sha-shared
+  event: push
+  event_payload: '{"head_commit":{"id":"sha-shared"}}'
+-
+  id: 1991
+  repo_id: 100
+  index: 9
+  commit_sha: sha-other
+  event: release