Omit any file that is not a zip when repackaging

Rather than using an artifact's type to determine whether or not it should be included, we now look at the file's header to determine if it's a zip file. A pleasant side-effect of this change is that the logic for deciding what should be included is now in a single location, `Repackager`, rather than being split across the two `Libraries` implementations in the Maven and Gradle plugins.