[6/x] Mirroring nested debian structure

[likhitha77]

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

DO NOT MERGE UNTIL COMPLETELY READY

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How to test:

To test this, we need to add a custom Debian repo with a nested structure. We can use this: http://ppa.launchpadcontent.net/ondrej/nginx/ubuntu/dists/focal/

> bin/rmt-cli repos custom repos add http://ppa.launchpadcontent.net/ondrej/nginx/ubuntu/dists/focal/ debian-nested

> bin/rmt-cli mirror repository debian-nested

• The mirror command should succeed without errors.
• We also need to check that the contents are mirrored to the correct location.
  • Please check that all the files listed here are present in the public/repo/ondrej/nginx/ubuntu/dists/focal path.
  • The pool directory contents must be present within the public/repo/ondrej/nginx/ubuntu/pool path

Additional check : Running the mirror command again should not throw any errors.