Support New Syntax to Declare Multiple Versions of the Same Dependency

_**NOTE: Proposed syntax has changed since this issue was reported. See latest comment below.**_

Some IGs may need to support multiple versions of the same dependency package. To support this, the following syntax will be used:
```
"hl7.fhir.us.core-old" : "npm:hl7.fhir.us.core@3.1.1",
"hl7.fhir.us.core" : "6.1.0",
```

SUSHI has been identified as a core tool that will need to support this.

For more information, see: https://chat.fhir.org/#narrow/stream/179239-tooling/topic/Advance.20Notice/near/439946498