pkgs/development/compilers/ghc/with-packages.nix: add 'ignoreCollisions' parameter

The ghcWithPackage expression now has an argument 'ignoreCollisions' that
allows users to disable the path collision check like so:

  (pkgs.haskellPackages.ghcWithPackages (pkgs: with pkgs; [ haskellPlatform ])).override { ignoreCollisions = true; };

See https://github.com/NixOS/nixpkgs/commit/d64917ad17277b8e16893ee31533ec2e33446fa7
for a long and detailed discussion of why these path collisions may occur.

pkgs/development/compilers/ghc/with-packages.nix

@@ -1,4 +1,4 @@
-{ stdenv, ghc, packages, buildEnv, makeWrapper }:
+{ stdenv, ghc, packages, buildEnv, makeWrapper, ignoreCollisions ? false }:
 
 assert packages != [];
 
@@ -12,6 +12,7 @@ in
 buildEnv {
   name = "haskell-env-${ghc.name}";
   paths = stdenv.lib.filter isHaskellPkg (stdenv.lib.closePropagation packages) ++ [ghc];
+  inherit ignoreCollisions;
   postBuild = ''
     . ${makeWrapper}/nix-support/setup-hook
 

pkgs/top-level/haskell-packages.nix

@@ -102,8 +102,9 @@ let result = let callPackage = x : y : modifyPrio (newScope result.finalReturn x
   # argument to this function.
 
   ghcWithPackages = pkgs : callPackage ../development/compilers/ghc/with-packages.nix {
-    ghc = ghc; # refers to ghcPlain
+    ghc = ghc;                  # refers to ghcPlain
     packages = pkgs self;
+    ignoreCollisions = false;
   };
 
   ghcWithPackagesOld = pkgs : callPackage ../development/compilers/ghc/with-packages-old.nix {