message: Use command-line xz when available
  Summary:
  Python's lzma support does not include multithreading. While the packer is parallelized across entries, single entries might be large, and parallelization may be benficial.
  NOTE: Compression ratio degrades a bit with parallel compression.
diff: @@ -15,6 +15,7 @@ import lzma
  import multiprocessing
  import os
  import shutil
  +import subprocess
  import tarfile
  import tempfile
  import timeit
  @@ -132,6 +133,12 @@ class _TarGzCompressor(_Compressor):
  def _compress_xz(from_file: str, to_file: str) -> None:
  + if shutil.which("xz"):
  + logging.debug("Using command line xz")
  + cmd = f'cat "{from_file}" | xz -z -7e -T10 - > "{to_file}"'
  + subprocess.check_call(cmd, shell=True) # noqa: P204
  + return
  +
  with lzma.open(filename=to_file, mode="wb", preset=7 | lzma.PRESET_EXTREME) as xz:
  with open(from_file, "rb") as f_in:
  shutil.copyfileobj(f_in, xz)
  