Roll out horizon-flake-parts 0.19.0 to package set repos with horizon.packageSet readme generation

Roll out horizon-flake-parts 0.19.0 to package set repos:

package-sets/horizon-build-packages (all lts/ghc-* branches)
package-sets/horizon-core (all lts/ghc-* branches)

Use horizon.packageSet for README generation.

Follows from #7 (closed) (0.18.0 rollout, now closed).