Use this as a default.nix in the root servant folder to aggregate all dependencies

default.nix

# This is a function: we take `haskellPackages` as an argument so that it can be overriden,
# but supply `nixpkgs.haskellPackages` as a default value if no value is explictly given.
{ haskellPackages ? (import <nixpkgs> {}).haskellPackages }:

let
  # Build our own haskell package set with all the packages added that we want
  myHaskellPackages = haskellPackages.override {

    # super refers to the "previous" haskell package set (the one we extend)
    # self refers to the final haskell package set, after our customizations have been
    # applied. 
    #
    # For example, if `myHaskellPackages` is extend again with some package 
    # `foo`, then that package would not be present in `super` but it would be present in
    # `self`.
    overrides = self: super: {
    
      # Add all the packages
      servant = self.callPackage ./servant {};
      servant-client = self.callPackage ./servant-client {};
      # ...
    };

  }

  # Now, build a "meta" package that has all the packages we want:
  aggregated = myHaskellPackages.mkDerivation {
    pname = "servant-aggregated";
    version = "0.0.1";
    src = null;
    license = null;
    libraryHaskellDepends = with myHaskellPackages; [ 
      servant servant-client ghc-mod
    ]; # ... 
    librarySystemDepends = [ ]; # add native stuff here
  };

# and use it for our env
in aggregated.env