Configurable version policy for resources installed by package installer

[mrdnctrk]

We used to keep the client-assigned fhir ids for IG package resources STORED in the repository. But this made it impossible to have multiple versions of US Core installed at the same time, which breaks multi-version validation. To fix this, there was a change for rules for package loading to match on canonical url+version, and use server assigned ids. This change has broken users who depended on the old behaviour.

This issue is about making this behavior configurable whether to allow installing resources with multiple versions from a package or just allowing a single version as before.