Add dedicated ID types to avoid opaque strings (#642)
This allows us to enforce type safety within the resolver. For example,
in the index, we can remove `String` as a key type and enforce that
callers _must_ present us with a `PackageId`. (This actually caught one
bug, where we were using the SHA rather than the package ID. That bug
shouldn't have had any effect given where it was, since those are 1:1,
but it's still problematic.)
