Upstream work-on-multi into Nixpkgs's shellFor

[Ericson2314]

See definition of shellFor in https://github.com/NixOS/nixpkgs/blob/8e4c9d15456fd3a35044bcea864d558fade4a021/pkgs/development/haskell-modules/make-package-set.nix#L253-L323 . It works today by splitting the dependencies as "haskell" vs "system", and then making a shell based off of this.

This, however, is too coarse-grained. We should instead first combine all the packages as work-ont-multi does. In the cross case we should also make two ghcWithPackages calls, one for Setup.hs with setup depends and buildHaskellPackages.ghcWithPackages in addition to the regular one. In the native case, we should combine regular haskell dependencies with setup-depends for one big ghcWithPackages call.

[Ericson2314]

To anyone working on this, take a glance at https://github.com/NixOS/nixpkgs/pulls?utf8=%E2%9C%93&q=is%3Apr+shellFor+ for the history of shellFor, and NixOS/nixpkgs#46453 in particular.

I think the plan should be to somewhat undo NixOS/nixpkgs@51267d3 from that PR, in that .env will not use shellFor. Instead, shellFor would use .env: The logic from shellFor that is moved back inside generic-builder to define the .env, and the remaining bits of shellFor are replaced with work-on-multi, combining the packages into one package from which to get and use a .env.

[Ericson2314]

Nixpkgs side is is in NixOS/nixpkgs#76349. After that's merged and we switch to 19.09, we can then use it from reflex-platform.