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>

docs/build-source.md

@@ -53,10 +53,10 @@ For packagers who need to use paths like `/etc/gitea/app.ini`,
 they should define these values at build time for `make build` by environment variable like
 `LDFLAGS='-X "module.Var1=Value1" -X "module.Var2=Value2"' TAGS="bindata" make build`.
 
-- _`CustomConf`_: `-X "code.gitea.io/gitea/modules/setting.CustomConf=/etc/gitea/app.ini"`
-- _`AppWorkPath`_: `-X "code.gitea.io/gitea/modules/setting.AppWorkPath=/var/lib/gitea"`
-- _`CustomPath`_: `-X "code.gitea.io/gitea/modules/setting.CustomPath=/var/lib/gitea/custom"`
-- Default PID file location: `-X "code.gitea.io/gitea/cmd.PIDFile=/run/gitea.pid"`
+- _`CustomConf`_: `-X "gitea.dev/modules/setting.CustomConf=/etc/gitea/app.ini"`
+- _`AppWorkPath`_: `-X "gitea.dev/modules/setting.AppWorkPath=/var/lib/gitea"`
+- _`CustomPath`_: `-X "gitea.dev/modules/setting.CustomPath=/var/lib/gitea/custom"`
+- Default PID file location: `-X "gitea.dev/cmd.PIDFile=/run/gitea.pid"`
 
 Add as many of the strings with their preceding `-X` to the `LDFLAGS` variable and run `make build`
 with the appropriate `TAGS` as above.