Use apt-ftparchive instead of reprepro

Use apt-ftparchive instead of reprepro

Afaict, this should work quite similar to the previous dpkg-scanpackages but
also doesn't use perl and is part of apt. Certain distros don't have reprepro
packaged, e.g. nixpkgs. It's also a simpler to work with and smaller compared
to reprepro.