Add additional attrs to enable source lookup for AetherRepo artifacts

In order to properly support source artifact lookup for aether based repositorys, we need to encode bsn and version attributes into IClasspathEntries so they can be later retreived by repository plugins that wish to look up source bundles 

This PR is here to support the source artifact lookup PR in the AetherRepository plugin in bnd here: https://github.com/bndtools/bnd/pull/595