haskell-modules: refactor package set

This change is effectively a no-op to nixpkgs. However, it gives users
the flexibility to create and maintain their own package sets per
project, while benefiting from nix's Haskell configurations.

I would make immediate use of this change in stack2nix, a project that
generates nix expressions for all the dependencies of a given Haskell
project. @domenkozar is familiar with the motivations and helped
refine this change

pkgs/development/haskell-modules/default.nix

@@ -2,6 +2,7 @@
 , compilerConfig ? (self: super: {})
 , packageSetConfig ? (self: super: {})
 , overrides ? (self: super: {})
+, initialPackages ? import ./hackage-packages.nix
 }:
 
 let
@@ -10,7 +11,7 @@ let
   inherit (import ./lib.nix { inherit pkgs; }) overrideCabal makePackageSet;
 
   haskellPackages = makePackageSet {
-    package-set = import ./hackage-packages.nix;
+    package-set = initialPackages;
     inherit ghc;
   };