Support FHIR-release-specific "current" build dependencies

The package loader currently cannot find some FHIR-release-specific dependencies (for example, `hl7.fhir.uv.extensions.r4#current`). This is because that package is not explicitly listed in the [qas.json](https://build.fhir.org/ig/qas.json) file.

Add support for this by using the following approach:
1. Look for the dependency as-is (e.g., `hl7.fhir.uv.extensions.r4`); if found, use it; otherwise...
2. If the package id ends with an `r` number (e.g., matches `/\.r\d+$/`), remove the end and search for the base package (e.g., `hl7.fhir.uv.extensions`); if found...
3. Determine the package build folder on the build server per the normal approach and try to download a tgz w/ the name of the originally requested package (e.g., `https://build.fhir.org/ig/HL7/fhir-extensions/branches/master/hl7.fhir.uv.extensions.r4.tgz`). If found, use it; otherwise...
4. Download the base package (without the `r[1-9]` ending) as normal, but log a warning.

(I'm not sure about step 4, but it's worth considering. We could also check the package metadata to confirm if it is for the right FHIR release an only download it if it is. Or we could just give up).