Support Special Rules for Contained CodeSystems in ValueSets

[cmoesel]

Based on this Zulip conversation, it is invalid for ValueSet.compose.include.system (as well as its exclude variant) to set the uri value to a fragment (e.g., #foo). This means that the standard way of referencing a contained CodeSystem will not work.

Instead:

1. Use the contained CodeSystem's full canonical URL as the value of ValueSet.compose.include.system
2. Put a valueset-system extension on ValueSet.compose.include.system and set its valueCanonical to the fragment pointing to the id of the contained CodeSystem (e.g., #foo)

Authors can do this the hard way right now (setting the values directly), but I suspect if they use Canonical(ContainedCodeSystem), it might do the wrong thing and directly insert a fragment. We should try to detect this situation and do the right thing:

• At the least don't convert the ValueSet.compose.{include|exclude}.system value to a fragment, even when there is a matching contained CodeSystem.
• Maybe go the next step and insert the extension if it has not been manually added already.

Also see: https://jira.hl7.org/browse/FHIR-44896

[mint-thompson]

Add to backlog as https://standardhealthrecord.atlassian.net/browse/CIMPL-1246