Roll out horizon-flake-parts 0.19.0 to horizon-ghc with horizon.ghc readme generation
Roll out horizon-flake-parts 0.19.0 to all ghc/horizon-ghc LTS branches. As part of this rollout, change ghc.common.hadrian to horizon.ghc so that it prints the README too.
Follows from #7 (closed) (0.18.0 rollout, now closed).