Add Packages.xz support

[j0hax]

Description

Hello there! I noticed that the main Debian Bookworm repository works well with RMT, but the Debian-Security repository only copies dists/ without the pool/ directory which contains the packages. Upon inspection of the RMT code I realized that only Packages.gz files are parsed, which are not contained in the Debian-Security repo.

How to test

Mirror http://security.debian.org/debian-security/dists/bookworm-security/main/binary-all/ or any repository which uses the .xz variant over Packages.gz

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[j0hax]

Also, the Debian Repository Format states:

Servers should offer only xz compressed files, except for the special cases listed above. Some historical clients may only understand gzip compression, if these need to be supported, gzip-compressed files may be offered as well.

I believe parsing both formats should be fine for now, but maybe it would be an idea to (at some point in the far future) phase out reading Packages.gz?

[j0hax]

Friendly ping to @ngetahun

[digitaltom]

Could you please fix the lint issue (rubocop), and make sure the unit tests are green (rspec). Thank you :-)

[ngetahun]

@j0hax Please extend existing unit tests. Do you have repos in mind still using xz?