Use root package by default for Stack

[qrilka]

packages is optional in stack.yaml but if I try stack-to-nix on such a project I get a failure:

stack-to-nix: nix/pkgs.nix: openFile: does not exist (No such file or directory)

[angerman]

@qrilka isn't the source of the error the lack of nix folder? I'd rather we just create the nix folder if it's missing, than move the pkgs file around.

The reasoning is the following: if someone starts adding packages, things would stay in place instead of moving around then and there.

[qrilka]

That missing directory error is just a side-effect of having no packages. We could create it sure but that means that we'll have just default.nix and pkgs.nix as end result - there will be no nix derivation for the root package itself. And that is not correct if the aim is to replicate what you'd have with Stack - stack build works fine for such a project and produces any components that the root package has.

[angerman]

@qrilka ahh! Sorry I missed that. So what you say is that stack defaults to . as the empty package set if no packages is specified.

LGTM.