Update FHIR Package Loader as necessary to support use in frontend web applications

[mint-thompson]

Transferred from internal JIRA issue originally created 2022-09-08.

Ideally, FSH Online and future web applications should use the same basic package loading infrastructure as SUSHI, GoFSH, etc. Update FPL to support this type of use.

Note that at the time of opening this issue, there is a large refactor in process for FPL. This task should not be started until that refactor is completed.