Support for other namespaces

In #165, @frasertweedale wrote > We are referencing the Hackage namespace. In most cases, the package description contains a reference to the repository. Is there any chance that there could be a metadata about the package that references the namespace? I'm currently working on Flora integration of advisories and I'd love to be able to fully disambiguate packages hosted by Hackage from package of the Cardano package repository. Right now I will restrict the advisories to the `@hackage` namespace, but I'm be delighted to see a field for the namespace / package repository one day. :)