Add validation check for FHIR IGs that raises a warning about Identifier.system values that aren't registered in THO

[lmckenzi]

Usual exclusions for systems routed in "example.org". Other than that, all systems should exist as identifier NamingSystems in THO.

[lmckenzi]

More specifically look for NamingSystems in the current IG or any dependencies and if you can't find the system URI defined there, spit out a warning.

We want this in the publisher for examples, but not in regular validation as in production there will be all sorts of Identifier.systems that don't exist as NamingSystems.