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.