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.