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

Change Type

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• [ ] Bug Fix (a non-breaking change which fixes an issue)
• [x] New Feature (a non-breaking change which adds new functionality)
• [ ] Documentation Update (a change which only updates documentation)

Checklist

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• [x] I have reviewed my own code and believe that it's ready for an external review.
• [ ] I have provided comments for any hard-to-understand code.
• [ ] I have documented the MANUAL.md file with any changes to the user experience.
• [ ] If my changes are non-trivial, I have added a changelog entry to notify users at package/obs/rmt-server.changes.

Review

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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