Avoid double Maven lifecycle run for building src and bin distributions

Currently the build goes through two invocations of mvn package assembly:directory, one for building the deploy-artifacts structure and the other for creating the actual distribution. While it's currently not possible yet with our maven project structure to have multiple executions of the assembly plugin in the same run, we can in any case have the plugin run multiple assembly descriptors.
This should basically save some time while building src and bin distributions.