cerbero/package: New argument --xz to output .tar.xz

This option is only used when the package command will create a tarball or an Android package (which inherits from DistTarball).

Uses xz --threads=0 by default to maximize parallelism. This was compared with pixz and found to be either the same or better in tests.

Benchmarks

See also !139 (merged)

xz -c config/cross-android-universal.cbc package -f --xz:

real    37m50.284s
user    37m31.323s
sys     0m15.858s

xz --threads=0 -c config/cross-android-universal.cbc package -f --xz:

real  14m22.177s
user  53m24.856s
sys  0m10.185s

pixz -c config/cross-android-universal.cbc package -f --xz:

real    15m10.181s
user    53m21.865s
sys     0m13.408s

Android Universal tarball sizes

558M gstreamer-1.0-android-universal-1.15.2.tar.bz2
372M gstreamer-1.0-android-universal-1.15.2.tar.xz