Add haskell-nix.project (and project')

[hamishmack]

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";`

[michaelpj]

project seems a little generic for me, maybe autoProject?

[angerman]

project seems a little generic for me, maybe autoProject?

@michaelpj you think? I would assume most people think of their haskell project? autoProject makes me wonder what the auto is all about?

[angerman]

lgtm

[domenkozar]

lgtm

[michaelpj]

I guess I just find project a bit non-descriptive. We have some haskell.nix nomenclature where we refer to a stack/cabal build as a "project", and so "project" makes sense as an abstraction over "cabal project" and "stack project". We're all used to this so it makes sense, but to a newcomer I worry that "project" is a bit too terse. Especially since we live in the Nix world where we have no types! TBH I find cabalProject and friends also too terse - if I had my way I'd probably call this mkPkgSetFromProject or something lengthy.