Regression: in some cases aggregator mojo is unable to resolve dependencies with custom packaging

As described in SONAR-2626 : aggregator mojo, which requires dependency resolution (like "sonar:sonar" or "javadoc:aggregate-jar") is unable to resolve dependencies, when executed from command-line and build extension with custom packaging declared in sub-module, but not in parent. Declaration of extension in parent allows to workaround problem as well as downgrade to Maven 2.2.1 and execution of "mvn validate sonar:sonar".