Meta issue to sum up what would need to be done in case anyone has enough interest in this to make it happen.

• (Optional) A new Bzip2Downloader should be added
• GzipDownloader and Bzip2Downloader's implementations of the extract method should be extracted in Composer\Util\Gzip|Bzip2 classes to make them reusable
• Add support for decompressing tar archives using those util classes in TarDownloader (Fixes "tar" type repository does not allow .tar.bz2 archives #6916 if Bzip2 was added)
• Once we have workable tar, see if it is possible/sensible to add a global config option prefer-archive (zip, tar, tar.bz2, tar.gz). If this is set to something non-zip, we could rewrite gitlab/github URLs from zip ones to tar.gz ones, or tar.gz for gitlab if available. Alternative to this would be to change what kind of URLs the VcsDriver outputs but then it isn't really selectable by the user anymore. I feel like keeping the zip default would probably be safest to avoid troubles. (Fixes add gitlab-dist-format option #6714)