Source of packages should be recorded in the manifest

[KristofferC]

Otherwise, we need the same registries that the manifest was created from and that is not recorded.

[greimel]

Having this would be really helpful for people that don't want to clutter General but also don't want to destroy reproducibility when sharing code. It's really easy to forget that a package from my private registry has been used and then my students or collaborators (not necessarily aware of package registries) get frustrated and cannot use my code.

(In order to avoid that I keep copy-pasting functionality across my teaching notebooks.)