Virtual Packages and SUSHI Compatibility Changes

[cmoesel]

Description: This PR includes various changes to better support the needs of SUSHI while remaining neutral for other environments and applications. These changes include:

• VirtualPackage interface for pseudo packages not in the registry or local FHIR cache
• DiskBased and InMemory implementations of Virtual Package
  • This replaces the previous local resource approach that used a special LOCAL#LOCAL package
  • In SUSHI, DiskBasedVirtualPackage is used to load local resources in input/resources, input/examples, etc
  • In SUSHI, InMemoryVirtualPackage is used to load the few R5 resources that should be resolvable in R4 (e.g., Base)
• Support for loading VirtualPackages in the BasePackageLoader
• Support for '|version' when searching by key
  • NOTE: I'm torn on whether we should support a single key argument that allows canonical version (e.g., findResourceJSON('SomeResource|1.2.3'), like in this PR) or if we should make version an optional 2nd argument (e.g., findResourceJSON('SomeResource', '1.2.3'), but then you need to pass in null if you don't want a version but need to pass in options as the 3rd argument)or make version an option (e.g., findResourceJSON('SomeResource', { version: '1.2.3' })).
• New exportDB function in SQLJSPackageDB and BasePackageLoader for debugging purposes.
• Added sort to find functions' options, with initial ByLoadOrder and ByType implementations (the original type argument is now just a filter, no longer affecting sort)
• Added resolveVersion function to RegistryClient interface and corresponding implementations
• Moved LRU cache from DiskBasedPackageCache to BasePackageLoader

Testing Instructions:

• Run the tests and ensure they pass.
• Run the CLI and use the --export option to export the SQLite DB to a file that you can load in a SQLite browser.
  • e.g., ts-node src/app.ts install hl7.fhir.us.core#7.0.0 hl7.fhir.us.mcode#2.0.0 --export
• If you wish to try it in SUSHI, it's already integrated into SUSHI's new-fpl branch (but note that I'm working on polishing that branch; tests are still breaking and I want to do some refactoring).