ContentPackage2FeatureModelConverter.java should implement Closeable

Forgetting to call cleanup on the ContentPackage2FeatureModelConverter leads to big dangling temp folders. To allow semantic code checks detect such failures, the class should implement Closeable and call cleanup in its close method.

Look at https://github.com/adobe/aemanalyser-maven-plugin/issues/76 for a concrete issue caused by this.