add a flag to prevent error when using only annotations: no descriptors found
extraction is bind to generate-sources in maven core for maven-plugin packaging so at this stage no classes available for scanning annotations
git-svn-id: https://svn.apache.org/repos/asf/maven/plugin-tools/branches/MPLUGIN-189@1333847 13f79535-47bb-0310-9956-ffa450edef68