[3/x] Refactor suma mirroring

[ngetahun]

This PR is part of our code sprint: https://trello.com/c/hU27QHob/3134-rdeb-sprint-tracker

The whole changeset is:

[1/5] Prepare code sprint and implement repository type detection
[2/5] Add flat debian mirroring support to RMT
[3/5] Refactor suma mirroring
[4/5] Refactor repomd mirroring
[5/5] Fixes and cleanup
[6/6] Nested debian repository support

Implementation in this merge request:

• Refactoring SUMA product tree mirroring into its own mirror class
• Remove product tree mirroring from repomd and make tests green

DO NOT MERGE UNTIL COMPLETELY READY

How to test this pull request:

$ mv public/suma/product_tree.json public/suma/product_tree.old.json
$ bin/rmt-cli mirror
# expect: You should see a log line indicating downloading the product tree

$ diff public/suma/product_tree.json public_suma/product.tree.old.json
$ expect: It should if at all show a new product added but otherwise the same file

Thanks for reviewing this pull request!

If you have any question please do not hesitate to approach me! 🚀

[suse-tests-pass]

Well Done! Your tests are still passing.
https://ci.suse.de/job/scc-RMT-integration-tests/354752/console
If the given link has expired,you can force a Prophet rerun by just deleting this comment. (Merged 41344af into 024478b)