Generalized version of the `cabalProject` and `stackProject`
functions.
Automatically selects the correct project type based on the
`projectFileName` argument if provided or what files exist in
the `src` directory if no `projectFileName` was specified.
If it cannot be determined which it should use the following
error message is given:
```
error: haskell-nix.project : both `stack.yaml` and `cabal.project` files exist set `projectFileName = "stack.yaml;"` or `projectFileName = "cabal.project";`
```
Yvan Sraka
John A. Lotoski
Hamish Mackenzie
Domen Kožar