Support for Multiple Versions of Same Dependency Package

[cmoesel]

SUSHI needs to support an IG declaring dependencies on multiple versions of the same package. This will require:

• a change to sushi-config.yaml to support specifying multiple versions of the same package
• a change to the package loader to support loading and resolving multiple versions of the same package (note: resolution may be complicated)

See:

• https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Multiple.20versions.20of.20a.20dependency/near/429220990
• https://chat.fhir.org/#narrow/stream/179192-fmg/topic/Publishing.20IGs.20that.20depend.20on.20multiple.20versions.20of.20US.20Core/near/422681201

[cmoesel]

There has been additional conversation regarding the right approach and expectations for this. It's not clear if sushi-config.yaml really does need to support specifying multiple versions of the same dependency package.

[cmoesel]

This is superseded by #1463.