Allow hook/override in PackagingDataCalculator

I'm running Logback as my SLF4J implementation in an OSGi container (Karaf). I love the feature of ThrowableProxy that populates stacktraces with the jar/version. Under Karaf, unfortunately, that data is always wrong because Karaf processes the jars into its data/generated-bundles folder before loading.

So, I forked PackagingDataCalculator in my own code to some class resolution logic (attached). That code is not important because it's too OSGi-specific for inclusion into Logback. But I'd like the ability to inject something like that calculation code into Logback so I don't have to fork PackagingDataCalculator any longer.