Default for packages option should use cabal.project if it exists

[srid]

... instead of the current */*.cabal (or *.cabal) globbing.

The algorithm would then be roughly: check for cabal.project or stack.yaml or *.cabal in the root and use that to set the packages option; if not present, require the user to set that option (or some helper option that sets it, for instance an option that specifies the location of a cabal.project file).

An error message should also be provided when the user hasn't set this option and their project has multiple-packages (no top-level .cabal) but without a cabal.project.

Originally posted by @roberth in #49 (comment)

[srid]

Regarding cabal.project parser: #49 (comment)

Haskell.nix has one, but I don't know how easily reused it is. Maybe the relevant paths can be found without much effort, I don't know. Dream2nix works with different files it seems. Maybe those contain all the needed info, but I don't think we should require them.

[ParetoOptimalDev]

I was going make https://github.com/haskell/cabal a Nix flake using haskell-template today to investigate the required effort for fixing haskell/cabal#8434 and ran nix flake init -t github:srid/haskell-flake and got an error.

I think it's because this isn't implemented yet?

If so, maybe https://github.com/haskell/cabal is a good candidate to be the golden test for this feature?

[srid]

Cabal is a big project, maybe something simpler can be used to test this feature. In regards to nixifying https://github.com/haskell/cabal using haskell-flake, @ParetoOptimalDev, I'd suggest you start a post here since I'm sure there will be more to it than cabal.project support; thus general discussion about it in #76 could become somewhat off topic.

[srid]

https://github.com/srid/haskell-flake/tree/master/nix/find-haskell-paths