try xz instead of bz2 first for compressed files

xz has pretty much won "the compressor war" and e.g. the Debian archive
doesn't even distribute bz2 anymore in favor of 'xz' and 'gz', so by
changing the default order we have a more realistic --print-uris
behavior as it will always show the first compressor.

In practice this effects repositories without a Release file (very bad,
we don't want to support them anymore anyhow) as xz will be tried before
bz2 now [which is probably not available, but so might be bz2…] AND
repositories which provide both, bz2 and xz (which isn't too common) in
sofar as apt will now download xz instead of bz2.

Users with special needs can stick with bz2 as first compressor tried
with Acquire::CompressionTypes::Order:: "bz2"; (see man apt.conf) – but
users with special needs usually prefer "gz" anyhow, so the realworld
change is expected to be very low.

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