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

* moving maven-cayenne-modeler-plugin to plugins folder to align with its Maven group

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1542480 13f79535-47bb-0310-9956-ffa450edef68
CAY-1883 - Clean up Cayenne maven structure - get rid of aggregate modules

* extracting plugins dep version management to the parent pom

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1542481 13f79535-47bb-0310-9956-ffa450edef68