Document attribute ordering in package expressions

Development Documentation
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I like having consistent ordering so I can quickly locate the things I need to change. We should also mention rationales for the more controversial parts – for example, the one per line rule is to make git diffs, as well as merges cleaner.

I think adding that example should be enough, people are pretty good at pattern matching. And adding an opinionated second level to nixkpgs-fmt would help a lot. Ideally, people should not have to think about this much.

Regarding the actual ordering, I like to order the attributes in meta alphabetically. Thankfully, it keeps the common one reasonably ordered (project info on top, nixpkgs-only stuff on the bottom, though the platforms kind of belong to the project too, now that I am thinking about it).

Few more things we might want to show in the example:

  • fetchFromGitHub (though the ordering of arguments is pretty intuitive)

  • environment variables like NIX_CXX_FLAGS (__structuredAttrs will mandate placing them into env attribute)