To depend on KEGGREST or not?

[Aariq]

@KristinaRiemer made a PR to add some functionality to KEGGREST that was needed for volcalc in 2021 and it hasn't been merged yet. In the meantime, we've used httr2 to access the KEGG API. Given that we aren't doing much with the KEGG API (just getting compound IDs from pathway IDs and downloading .mol files), we could just do all of that by directly accessing the KEGG API.

Pros: one fewer dependency, we are already doing one of the two things without the KEGGREST package
Cons: we would then be responsible for fixing things if the KEGG API changes (this could also be a 'pro' if, for example, the KEGGREST maintainers continue to be slow to merge contributions)