Separate attribute set construction from flake output construction

[cleeyv]

This refactoring is to make the flake simpler and easier to understand, and also to make it more tractable to switch to a default.nix for the repo, as proposed here: #209

Related to this is understand (and perhaps fixing) the current requirement for nixosModule outputs for project services to be in quotes, like ngipkgs.nixosModules."services.vula". It seems to break with usual conventions around nix syntax which seems more likely to result in confusion or errors.