Custom artifact handlers should be ignored

If a custom packaging has a custom artifact handler, that handler (in particular, the file extension) should be ignored by the M2 --> M1 plugin.

For example, if I create a packaging "foo" and use the artifact handler to declare that my packaging has the file extension "jar", then the plugin will currently create deploy files in the form "foos/a.jar". This is a corrupted repository from M1's point of view, which expects the packaging and the file extension to match in all cases.

The workaround is to use M1's <jar> dependency field to override the calculated file name, but the real issue should also be fixed.