Haskell infrastructure: fix #9871 (overrideCabal)

As per the recommendation by @Mathnerd314, this change seems to work
with the testing I did.

It makes sense to me why it works, but I think it's in an important
enough place that someone with much deeper knowledge of Haskell
infrastructure in Nixpkgs should give it a really heavy review.

I also consolidated all the overrideCabal definitions (there were two)
into a single definition in haskell-modules/lib.nix.

pkgs/development/haskell-modules/lib.nix

@@ -3,7 +3,7 @@
 rec {
 
   overrideCabal = drv: f: (drv.override (args: args // {
-    mkDerivation = drv: args.mkDerivation (drv // f drv);
+    mkDerivation = drv: (args.mkDerivation drv).override f;
   })) // {
     overrideScope = scope: overrideCabal (drv.overrideScope scope) f;
   };