CAY-1883 - Clean up Cayenne maven structure - get rid of aggregate modules

* renaming maven-cayenne-build-plugin to cayenne-build-maven-plugin to follow modern maven conventions

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1542372 13f79535-47bb-0310-9956-ffa450edef68