Changes to the haskell.nix code to fix broken tests:
* Add missing `then` to `call-cabal-project-to-nix.nix`.
* Fix default `hsSourceDirs` so that `.` gets included for `.all` component if one of the components does not have a `hsSourceDir` set.
* Fix `haskellNixRoots` so it works when cross compiling to windows.
Improvements to the haskell.nix tests:
* Run haskell.nix tests of nixpkgs 19.03 and 19.09.
* Run haskell.nix tests cross compiled to Windows under Wine (when possible).
* Add nix used as IFD inputs as tests to ensure they are cached.
* Use `haskell-nix.cabal-install` instead of `nixpkgs.cabal-install` in tests.
Project flags (from stack.yaml and plan.json) are exported in a modules attribute by stack-to-nix and plan-to-nix, but are not currently used. This change updates `mkStackPkgSet` and `mkCabalProjectPkgSet` so that the modules attribute is used (if present) and includes tests to check they are.
This commit makes `stdenv.lib.mkOverride` necessary for setting flags in `mkCabalProjectPkgSet` modules, however it also means that you can set them in `cabal.project` instead (see #254).
Hamish Mackenzie