Allow changing nixpkgs to update Haskell package versions

Stack2nix is cool if you have to maintain a `stack.yaml` and want a reliable way to turn it into a nix file. However it does not work well as a migration tool yet.

For instance here is the haskell-ide-engine [stack.yaml](https://github.com/haskell/haskell-ide-engine/blob/master/stack.yaml). Stack2nix outputs a 1.59MB [nix file](https://github.com/domenkozar/hie-nix/blob/master/ghc-8.4.nix) that pins all the haskell packages. This means you can't change nixpkgs to get different haskell packages versions (you will always get the ones stack would have chosen). This locks you into maintaining a stack.yaml file and running `stack2nix` (instead of bumping `nixpkgs` to a newer commit).

I think if `stack2nix` took a nixpkgs ref as an input it could get much closer to the hand crafted translation. It could compare the nixpkgs derivations to the ones it would have output and only output the differences.

Here is an example of a by hand translation and I think something like `stack2nix --migrate --nixpkgs ''` should be able to get pretty close to this:

with import { };

let
rootName = name: builtins.elemAt (lib.splitString "/" name) 0;
isValidFile = name: files: builtins.elem (rootName name) files;
relative = path: name: lib.removePrefix (toString path + "/") name;
onlyFiles = files: path: builtins.filterSource (name: type: isValidFile (relative path name) files) path;

ghc = (pkgs.haskell.packages.ghc843.extend (pkgs.haskell.lib.packageSourceOverrides {
haskell-ide-engine = onlyFiles ["LICENSE" "Setup.hs" "app" "haskell-ide-engine.cabal" "src" "test"] ./.;
hie-plugin-api = ./hie-plugin-api;
HaRe = ./submodules/HaRe;
cabal-helper = ./submodules/cabal-helper;
ghc-mod = ./submodules/ghc-mod;
ghc-mod-core = ./submodules/ghc-mod/core;
haskell-lsp = ./submodules/haskell-lsp;
haskell-lsp-types = ./submodules/haskell-lsp/haskell-lsp-types;
haskell-lsp-test = ./submodules/haskell-lsp-test;
leksah-server = ../leksah/vendor/leksah-server;
base-compat = "0.9.3";
cabal-plan = "0.3.0.0";
haddock-api = "2.20.0";
haddock-library = "1.6.0";
})).extend(self: super: {
hlint = haskell.lib.dontCheck super.hlint;
constrained-dynamic = haskell.lib.doJailbreak super.constrained-dynamic;
haddock-api = haskell.lib.dontCheck super.haddock-api;
haddock-library = haskell.lib.dontCheck (haskell.lib.dontHaddock super.haddock-library);
ghc-exactprint = haskell.lib.dontCheck super.ghc-exactprint;
leksah-server = haskell.lib.dontCheck super.leksah-server;
});
in {
inherit ghc;

shells = {
ghc = ghc.shellFor {
packages = p: [
p.haskell-ide-engine
p.hie-plugin-api
p.HaRe
p.cabal-helper
p.ghc-mod
p.ghc-mod-core
p.haskell-lsp
p.haskell-lsp-types
p.haskell-lsp-test
];
};
};
}

If `./` is included as package, the list of files passed to `onlyFiles` should be added to prevent non package files from triggering builds (I made that list by running `cabal sdist` and looking at the top level items in the `.tar.gz` file it produced).