Compiler: Text -> List Natural

compiler -> ghcVersion

horizon-spec/example.dhall

 let H =
       ./package.dhall
-        sha256:97fe5ca3cce90a439b4ffdddbdcad5e3383a8d4a88d2a55d34b49e32944e4f72
+        sha256:b27c8370f2ca1cf1b3d3465f5860dc78d67be1615dd9de94f091439c10008fff
 
 let packages =
       { Cabal = H.callHackage "Cabal" [ 3, 8, 1, 0 ]
@@ -154,5 +154,5 @@ let packages =
 in  H.HorizonExport.MakePackageSet
       { packageSetFile = "initial-packages.nix"
       , packagesDir = "pkgs/"
-      , packageSet = { compiler = "ghc-9.4.4", packages = toMap packages }
+      , packageSet = { ghcVersion = [ 9, 4, 4 ], packages = toMap packages }
       }