SUSHI does not work well w/ IG packages that don't include snapshots

While the HL7 IG Publisher includes snapshots in its packages, Simplifier does not. If a FSH project depends on a Simplifier package, this causes problems. We need to investigate approaches to making this easier on our users.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Dependencies

Start by reviewing how SUSHI handles FSH projects that depend on IG packages, comparing packages with and without snapshots. Read the linked chat.fhir.org discussion for existing context and proposed approaches. Done means identifying and documenting a practical approach that makes Simplifier packages without snapshots easier for users.

Written by the indexing model from the issue text.