haskell infra: Misc cleanups

- The haskell lib is very close to not relying on Nixpkgs. I think
   this is good---simpler to think about and matches Nixpkgs's lib.

 - The haskell lib is only imported once

 - stdenv is exposed more shallowly so it can be overriden more easily.
   I'll eventually use this on Darwin to avoid the Sierra shared
   library problems (unless changes are to be made system-wide).

Closes https://github.com/NixOS/nixpkgs/pull/27840.

pkgs/development/haskell-modules/make-package-set.nix

@@ -1,20 +1,34 @@
 # This expression takes a file like `hackage-packages.nix` and constructs
 # a full package set out of that.
 
-# required dependencies:
-{ pkgs, stdenv, all-cabal-hashes }:
+{ # package-set used for non-haskell dependencies (all of nixpkgs)
+  pkgs
 
-# arguments:
-#  * ghc package to use
-#  * package-set: a function that takes { pkgs, stdenv, callPackage } as first arg and `self` as second
-#  * extensible-self: the final, fully overriden package set usable with the nixpkgs fixpoint overriding functionality
-{ ghc, package-set, extensible-self }:
+, # stdenv to use for building haskell packages
+  stdenv
+
+, haskellLib
+
+, # hashes for downloading Hackage packages
+  all-cabal-hashes
+
+, # compiler to use
+  ghc
+
+, # A function that takes `{ pkgs, stdenv, callPackage }` as the first arg and `self`
+  # as second, and returns a set of haskell packages
+  package-set
+
+, # The final, fully overriden package set usable with the nixpkgs fixpoint
+  # overriding functionality
+  extensible-self
+}:
 
 # return value: a function from self to the package set
 self: let
 
   inherit (stdenv.lib) fix' extends makeOverridable;
-  inherit (import ./lib.nix { inherit pkgs; }) overrideCabal;
+  inherit (haskellLib) overrideCabal;
 
   mkDerivationImpl = pkgs.callPackage ./generic-builder.nix {
     inherit stdenv;