Make some constants plain fields, not properties

Adding a modifier makes groovy not generate the getter. This avoids a
warning at runtime:

WARN  framework.Cglib2AopProxy  - Unable to proxy method [public static final java.lang.String org.transmartproject.db.support.MarshallerRegistrarService.getPACKAGE()] because it is final: All calls to this method via a proxy will be routed directly to the proxy.