Put subrepos in nested subdirectories

[korewaChino]

related to #3353

We should split repo streams in subdirectories similar to how Fedora's Pungi artifacts or Debian organizes repos, so we would have something like this:

terra43/
    main/
        x86_64/
        aarch64/
        src/
    extras/
        x86_64/
        aarch64/
        src/
    nvidia/
        x86_64/
        aarch64/
        src/
   mesa/
        x86_64/
        aarch64/
        src/
...

This, of course, just like #3353 would need extra care since we would require migration. However the impact should be somewhat minimal if Tetsudou can simply just provide the right metalinks

Reference from Pungi composes: https://dl.fedoraproject.org/pub/fedora/linux/releases/43/

[linear]

FYRA-429