In preparation for a unified release process, we need to be able to
generate the pom files independently of trying to actually publish. This
change adds back the maven-publish plugin just for that purpose. The
nexus plugin still exists for now, so that we do not break snapshots,
but that can be removed at a later time once snapshots are happenign
through the unified tools. Note I also changed the dir jars are written
into so that all our artifacts are under build/distributions.