Switch from xz-utils to liblzma in dpkg Pre-Depends

[Guillem Jover <guillem@debian.org>]

Hi!

As I'd like to change a Pre-Depends in dpkg, I'm bringing this up here
for discussion, as per policy §3.5 and given dpkg “Essential: yes”
nature.


As mentioned in [0] some time ago, I'd like to switch the Pre-Depends
from the current xz-utils commands to use the liblzma shared library,
(with a patch provided by Jonathan Nieder). This will reduce the
pseudo-essential package set and installed size slightly, and it
will also allow for speed up improvements in the future.

  [0] <http://lists.debian.org/debian-devel/2010/02/msg00511.html>

There might be packages now implicitly depending on xz-utils given
that it's being pulled by dpkg, but those are buggy and should be
fixed regardless.

If there's no strong reasons not to, I'd like to include this for
1.16.1, as part of the dpkg buffer api rework needed for multiarch,
the conffiledb and automatic checksum generation.

thanks,
guillem