simplify some of the nix code

[adrian-gierakowski]

would you be open to a PR simplifying some of the nix code, to make it more readable?

For example:

yarnpnp2nix/lib/mkYarnPackage.nix

Line 117 in ac8dc41

_outputHash = if builtins.hasAttr "outputHash" packageManifest && packageManifest.outputHash != null then packageManifest.outputHash else null;

could be rewritten to:

 _outputHash = packageManifest.outputHash or null;

same thing could be applied to most cases where hasAttr is used in the above file