fix: handle viewing a submodule entry (#7261)
- When trying to view a submodule directory via the normal `/src/branch/` path, generate a redirect link to the submodule location.
- Resolves forgejo/forgejo#5267
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7261
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Gnarwhal <git.aspect893@passmail.net>
Co-committed-by: Gnarwhal <git.aspect893@passmail.net>