WIP: Add SteamRT 1½ 'heavy'

This doesn't quite work yet, although it's already on the way to working: I need to change it so it will remove udev before removing sysv-rc.

$ http_proxy=http://192.168.122.1:3142 no_proxy=192.168.122.1 ~/src/flatdeb/run.py --suite=heavy --arch amd64 --replace-apt-source=steamrt='both [trusted=yes] http://steamos-master.valve.org/steamrt:/heavy:/main/main ./' --generate-sysroot-tarball base
...
$ http_proxy=http://192.168.122.1:3142 no_proxy=192.168.122.1 ~/src/flatdeb/run.py --suite=heavy --arch amd64 --replace-apt-source=steamrt='both [trusted=yes] http://steamos-master.valve.org/steamrt:/heavy:/main/main ./' --generate-sysroot-tarball runtimes runtimes/heavy.yaml
...
2019/03/01 18:21:24 platformize | + systemd-nspawn --directory=/scratch/root --as-pid2 env DEBIAN_FRONTEND=noninteractive SUDO_FORCE_REMOVE=yes no_proxy=192.168.122.1 http_proxy=http://192.168.122.1:3142 dpkg --purge --force-remove-essential --force-depends debconf debian-archive-keyring e2fsprogs init insserv login mount passwd systemd systemd-sysv sysv-rc udev perl-base
...
2019/03/01 18:21:25 platformize | /var/lib/dpkg/info/udev.postrm: 20: /var/lib/dpkg/info/udev.postrm: update-rc.d: not found