Generated stack2nix output file doesn't contain tree-diff

When running stack2nix 0.2 on a project using `resolver: lts-9.1` (succeeds) and then trying to build the output package:

```
error: anonymous function at /nix/store/c79p3krp7li3rnc6mfjaar0n82kkxf3k-cabal2nix-Cabal/default.nix:1:1 called without required argument 'tree-diff', at /nix/store/5nj7vgd7spdcvxs47gpv58nqjs76vsla-source/pkgs/development/haskell-modules/make-package-set.nix:88:27
```

```
% cat /nix/store/c79p3krp7li3rnc6mfjaar0n82kkxf3k-cabal2nix-Cabal/default.nix
{ mkDerivation, array, base, base-compat, base-orphans, binary
, bytestring, containers, deepseq, Diff, directory, filepath
, integer-logarithms, mtl, optparse-applicative, parsec, pretty
, process, QuickCheck, stdenv, tagged, tar, tasty, tasty-golden
, tasty-hunit, tasty-quickcheck, text, time, transformers
, tree-diff, unix
}:
mkDerivation {
pname = "Cabal";
version = "2.2.0.1";
src = /nix/store/z64nmjm5rlgdjk44qchkmia6mjhim8in-cabal-dedupe-src;
setupHaskellDepends = [ mtl parsec ];
libraryHaskellDepends = [
array base binary bytestring containers deepseq directory filepath
mtl parsec pretty process text time transformers unix
];
testHaskellDepends = [
array base base-compat base-orphans bytestring containers deepseq
Diff directory filepath integer-logarithms optparse-applicative
pretty process QuickCheck tagged tar tasty tasty-golden tasty-hunit
tasty-quickcheck text tree-diff
];
doCheck = false;
homepage = "http://www.haskell.org/cabal/";
description = "A framework for packaging Haskell software";
license = stdenv.lib.licenses.bsd3;
}
```

In this case, Cabal's test suite depends on `tree-diff` but the generated stack2nix `haskellPackages` output file doesn't contain `tree-diff`.