haskell: have a dedicated file for non Hackage packages

We had a few "overrides" in configuration-common.nix that were really
extensions. They introduced packages that weren't in hackage-packages.nix.

The advantage of having a dedicated file for these packages is that we can still
place Nix-specific overrides to these packages in configuration-nix.nix. We
weren't able do this before because configuration-nix.nix extended only the
packages from hackage-packages.nix.

pkgs/development/haskell-modules/default.nix

@@ -4,6 +4,7 @@
 , packageSetConfig ? (self: super: {})
 , overrides ? (self: super: {})
 , initialPackages ? import ./initial-packages.nix
+, nonHackagePackages ? import ./non-hackage-packages.nix
 , configurationCommon ? import ./configuration-common.nix
 , configurationNix ? import ./configuration-nix.nix
 }:
@@ -26,7 +27,9 @@ let
       (extends packageSetConfig
         (extends compilerConfig
           (extends commonConfiguration
-            (extends nixConfiguration haskellPackages)))));
+            (extends nixConfiguration
+              (extends nonHackagePackages
+                haskellPackages))))));
 
 in