mkosi: Replace submodules with our own thing

mkosi: Replace submodules with our own thing

Unfortunately, git submodules break in all sorts of ways:

- Various github workflows (dependabot, github pages) try to do a shallow
clone of git submodules which does not work at all when the git repository
is hosted on pagure (https://pagure.io/pagure/issue/5453,
https://github.com/dependabot/dependabot-core/issues/9391).
- If the git forge hosting the git repository uses SHA256, then it breaks our
usage of it as a submodule as SHA256 repositories cannot be used as submodules
in SHA1 repositories (src.opensuse.org moved to SHA256 which broke our usage of
opensuse's systemd spec as a submodule).
- git submodules completely break usage of git worktrees.
- ...

Let's avoid all these issues by just doing our own home grown implementation of
git submodules. We lose the automatic dependabot updates this way but since dependabot
fails to run more often that not with submodules we don't really lose anything.