Support for purging output directory before unpacking. If a new version ...

...of the artifact is available and part of the contents is deleted in this new version, it should be deleted after unpacking to avoid undesired side effects. E.g. if we deploy large test data (let's say > 1 GB ) we would like to unpack it only if there is a change in the data but anything deleted from the previous version should be removed too. purgingOutputDirectory provides this capability. The consumer should be careful to configure purging in a non-conflicting way which can end up with deleting data that won't be unpacked.(If two artifacts have the same output folder and one of them needs processing and purges the output folder, then the other one should be unpacked even if it is not updated)