update_repo: support ddeb files

[Totktonada]

https://wiki.debian.org/AutomaticDebugPackages

In brief, it is the new file extension for a deb package that contains only debug symbols.

The document above has a section regarding dak (debian repository management tool) rules:

• ddeb will have no override entries, so no priority or section.
• They may not show up in the control-file, nor have a long description.
• dak will accept $binary-dbgsym_$version_$arch.ddeb if a $binary_$version_$arch.{deb,udeb} is included in the same upload.
• ddeb will go into the component "debug", so will show up in ./pool/debug/ (The pool-prefix is pretty much hardcoded, so making it variable would need a lot of work). No other packages will be allowed in "debug".
• Standalone Release/Packages in ./debug/dists/.

I think we should at least consider the recommendation to place such packages to a separate 'archive' (a repository) to don't bloat the main repository.

[Totktonada]

We replaced update_repo.sh with rws, so let's continue in tarantool/rws#13.