I think this needs to stop being commented out, now that it's released.

Oh, actually this is also wrong: we need _F28_UPDATES_REPO and F28_MODULAR_UPDATES_REPO, don't we?

This presumably will require additional changes later to consume them properly.

Really better as a separate commit, even if the same PR.

This looks like it will require changes to how _write_lookup_caches() reads module info - though I'm not entirely sure how the modules from the modular updates repo should be merged with the modules from the base modular repo - I'd guess if there's a module with the same name:stream in the updates repo, it should replace the module in the base modular repo entirely.