Handle removed packages

[henrik]

We look for updates and add any new/changed packages, but we don't get rid of removed ones.

We could make API requests for lists of all existing packages and compare to the DB. This would be fewer requests than checking every single package endpoint. Make sure we don't miss anything between pagination pages in a way that means we accidentally delete. Same job as #13?

This would indirectly also handle renamed packages: we can see them as a removed package + an added one.