mkosi: Introduce packaging sources as submodules

mkosi: Introduce packaging sources as submodules

By always cloning the latest branch commit, we can't bisect properly
using mkosi as when bisecting wildly different packaging sources will
be used compared to when the commit was merged. By using submodules, we
track individual commits which means when bisecting the same packaging
sources will be used.

We use git submodules as dependabot has support for automatically making
PRs to update git submodules. This commit also includes the necessary
dependabot configuration to enable this.

We make ubuntu/debian use the same submodule instead of adding the debian
packaging sources twice by introducing a new $PKG_SUBDIR environment variable
and using it instead of $DISTRIBUTION.