Add `hsPkgsOverrides` to allow customizing the Haskell package set

lib.nix

@@ -10,13 +10,16 @@
 # ^ A string giving the version of ghc and related packages to use.
 
 , packageName
-  # ^ A string giving the name of the project/package being packaged.
+# ^ A string giving the name of the project/package being packaged.
+
+, hsPkgsOverrides ? hfinal: hprev: {}
+# An overlay for haskellPackages.
 }:
 let
   hsPkgs = pkgs.haskell.packages.${compilerVersion}.override {
     overrides = hfinal: hprev: {
       ${packageName} = hfinal.callCabal2nix packageName src {};
-    };
+    } // (hsPkgsOverrides hfinal hprev);
   };
 in
 rec {