Rework the output directory so that it is contained within the :target hierarchy

git-svn-id: https://svn.apache.org/repos/asf/buildr/trunk@1525837 13f79535-47bb-0310-9956-ffa450edef68
Ensure that jdk-15 is correctly set, even with projects working on old JVMs

git-svn-id: https://svn.apache.org/repos/asf/buildr/trunk@1525838 13f79535-47bb-0310-9956-ffa450edef68