Make haskell.nix and nixpkgs configurable

AFAIU in https://github.com/poscat0x04/hls-nix/blob/master/default.nix the source argument is configurable by the call site, however this source is specific to this project: it's a set containing haskell-nix as well as the different HLS sources (0.1 and master).

Users (e.g. me :smile: I encountered this issue this week-end) can't (easily) pass a custom Haskell.nix. E.g. we use a specific version of Haskell.nix in my project, and we don't want to use another version (which would recompile a lot of stuff) just for HLS.
In the same spirit, users can't (easily) pass a custom Nixpkgs, as default.nix forces to use nixpkgs-2003 from Haskell.nix

Proposal: make at least haskell.nix and Nixpkgs arguments in default.nix, e.g.:

{ source ? import ./source
, haskellNix ? import source.haskell-nix { }
, pkgs ? import haskellNix.sources.nixpkgs-2003 haskellNix.nixpkgsArgs
, tag ? "master"
, version ? "8.8.3"
}:

# Implementation doesn't change

With such arguments:

those who only want to customize the Haskell.nix version/overlays/configuration can
those who want full customization on the Nixpkgs can

Note: I am still fresh in the Nix world, so my proposal or code may be wrong/have flaws that I'm not aware of!

poscat0x04 / hls-nix

Make haskell.nix and nixpkgs configurable #7