Build distribution packages in mkosi

Build distribution packages in mkosi

Instead of running meson install and hoping for the best, let's build
distribution packages from the downstream packaging specs. This gets
us the following:

- Vastly simplified mkosi scripts since we don't need a separate initrd
  image anymore but can just reuse the default mkosi initrd.
- Almost everything can move to the base image as its not the basis
  anymore for the initrd and as such we don't need to care about the
  size anymore.
- The systemd packages that get pulled in as dependencies of other
  packages get properly uninstalled and replaced with our packages that
  we built instead of just installing on top of an existing systemd
  installation with no guarantee that everything from that previous
  installation was removed.
- Much better testing coverage as what we're testing is much closer
  to what will actually be deployed in distributions.
- Immediate feedback if something we change breaks distribution packaging
- We get integration with the distribution for free as we'll automatically
  use the proper directories and such instead of having to hack this
  into a mkosi build script.
- ...