Various improvements such as:
1. Avoiding deduplications when there can't be any duplicates
2. Avoiding O(n^2) deduplications
3. Using builtins.any to avoid list allocations
4. Using builtins.concatMap instead of lib.flatten when it's known that there's only one level of nesting
5. Using builtins.groupBy instead of folding with an accumulator
In particular 5. should fix CI exceeding the stack size on staging: https://github.com/NixOS/nixpkgs/actions/runs/12989244871/job/36240781244?pr=377253
While 2. in particular should make CI a lot faster.
The handles can change over time and there's nothing guaranteeing the
ones in the maintainer list are up-to-date. In comparison GitHub IDs
never change.
Currently we need to rely on ofborg requesting reviews from package
maintainers, which takes a while with ofborg's eval queue. Since
recently we're doing faster evaluations with GitHub Actions, which contain all
necessary information to determine reviewers of changed packages the
same way ofborg does. This PR takes advantage of that.
