Move custom and patched plugins into submodules

Currently, most modules under packages/ are tracked within this repository.

This has remained until now because submodules of this project (a submodule itself in most projects) could only be defined using a relative repository URL. This would break CI in forked versions of the project.

However this has changed since GitLab Runner v15.11.0, and all submodules can now be defined using absolute URLs.

Having a distinct project for each of our custom or patched Lektor plugins will ease the task of keeping up with the various upstreams and allow for a more straightforward upstreaming workflow.