Update JpaPackageCache.java

[jkiddo]

Fix for #5834

[dotasek]

The fix looks correct, but there are a few things needed.

1. This wouldn't initially compile for me. I needed to add the following imports:

import org.hl7.fhir.instance.model.api.IBase;
import org.hl7.fhir.instance.model.api.IBaseBackboneElement;

2. If I understand the issue correctly, I think it would be possible to replicate the original failure in JpaPackageCacheTest with something similar to this:

try (InputStream stream = ClasspathUtil.loadResourceAsStream("/packages/hl7.fhir.uv.shorthand-0.12.0.tgz")) {				
  myPackageCacheManager.addPackageToCache("hl7.fhir.uv.shorthand", "0.12.0", stream, "hl7.fhir.uv.shorthand"); 
}

I was about to do so myself, but the IG in the original issue is not available: https://build.fhir.org/ig/WorldHealthOrganization/smart-anc/package.tgz

3. This needs a changelog entry before anyone authorizes the merge.

[jkiddo]

Odd ... I'll add some tests and the package as well

[jkiddo]

@dotasek all three bullets should now be adressed.