Auto merge of #5476 - dwijnand:Registry-vs-Source-confusion, r=matklad

Try & simplify Registry vs Source confusion

Refs #3006.

I tried to go further and replace references of `Registry` with `PackageRegistry`, but I ran into lifetime issue in `ops::resolve_with_previous` which I think has to do with no longer passing `PackageRegistry` as a `Registry` trait object (with a new and distinct lifetime).