This adds the convenience functions withDefaults and
withoutDefaults which simply wrap the provided argument in a list
either with or without the default poetry overrides.
Tests have also been adapted to make use of `withDefaults` where
appropriate.
This is not required for private repos where builtins.fetchGit would
suffice but _is_ required for Hydra (nixpkgs) where builtins.fetchGit is not allowed.
