I would recommend that you use a framework which distinguishes class loads. E.g. OSGi framework. Then you can create 2 bundles - one with the fizz implementation and one with the buzz implementation. They both can contain their dependent libraries which do not conflict anymore because they are loaded from 2 different classs loaders.
Example osgi containers or "Eclipse Equinox" and "Apache Felix" but there are more.
Hope that helps.