Consider whether SUSHI can or should detect when the special-url parameter is needed

[mint-thompson]

Transferred from internal JIRA issue originally created 2022-09-29.

A couple times now FSH users have needed to set a url on a CodeSystem that is not the default canonical URL. In this case, they can easily set the url in FSH, but the IG Publisher gives a validation error that the CodeSystem’s URL does not match the canonical URL. To resolve this error, users must use the special-url parameter in their IG resource. They can do this manually or using the sushi-config.yaml file if they use that to generate their IG resource.

However, it may be possible to have SUSHI detect this case and add the special-url parameter automatically. This task should do a few things, but it will not include implementation. Things to consider:

• Can SUSHI detect this case?
• Should SUSHI output a warning about it if we can detect it?
• Should we include the necessary special-url parameters in the IG resource if they are not specified in the config but needed?

Considering the factors above (and possibly others), this task should decide if this is something SUSHI should do.

Two relevant Zulip threads that brought this to our attention:

https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/SUSHI.20creating.20a.20problem.20with.20implementationGuide.2Ejson

https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/codesystem.20url