bsdtar, pkg_install and archivers/xz

[Steven Drake <sbd%NetBSD.org@localhost>]

While working on adding missing buildlinks to packages I add archivers/xz
to libarchive as it supports xz the same as it does zlib and bzip2.
However there is a problem with bsdtar and pkg_install which depend on
libarchive.

Libarchive and its dependences (currently only zlib and bzip2) are
distributed in the files directory of each package so bsdtar and
pkg_install can build and use them as part of the bootstrap kit/depends.

So I think archivers/xz should be treated the same and distributed in its
files directory.  I have the changes to xz, bsdtar and pkg_install tested
and ready to commit.

I case its of interest unpacked xz is about 4.8M.

If there are no objections I'll make this change an the end of the week.

-- 
Steven