private static FileSet mergeFileSet(final File tmpDir, final Artifact a) throws MojoExecutionException {
final File destDirectory = new File(tmpDir, "extract_" + a.getId().replace(':', '_'));
ModulizerUtil.mkdir(destDirectory);
final ZipUnArchiver unarchiver = unzipperForMerge();
unarchiver.setOverwrite(true);
unarchiver.setSourceFile(a.getFile());
unarchiver.setDestDirectory(destDirectory);
unarchiver.extract();
final DefaultFileSet files = new DefaultFileSet();
files.setDirectory(destDirectory);
return files;
}