Hey, I'm building some private Haskell packages.
  Package1 depends on 'a ver0'.
  Package2 depends on 'a ver1' and 'Package1'.

How do I build Package2 so that I can use functions exposed from the Package1
module?

*Current setup:*

```
let
  config = {
    allowUnfree = true;
    packageOverrides = pkgs: rec {
      haskellPackages = pkgs.haskellPackages.override {
        overrides = haskellPackagesNew: haskellPackagesOld: rec {
          package1 =
            haskellPackagesNew.callPackage ./nix/package1.nix { };

          package2 =
            haskellPackagesNew.callPackage ./cabal.nix { inherit package1; };
        };
      };
    };
  };

  pkgs = import <nixpkgs> { inherit config; };

in
  { package2 = pkgs.haskellPackages.package2;
  }
```

Where "./nix/package1.nix" and "./cabal.nix" are just generated from `cabal2nix`.

*Current output:*

```
... Building Package 1 ...
Setup: Encountered missing dependencies:
base >=4.6 && <4.9
```

In my case package 'a' is 'base'. Clearly the nixpkgs I import has a different
version of base (>4.10), which is the correct version for 'Package2' but not
'Package1'. Are these packages fundamentally not composable?