[RFC] Change repo layout from current/ to packages/<ARCH>/<LIBC>/

[0x5c]

This is my attempt to implement a new, more predictable repository layout, as previously discussed on IRC.

There are a number of open questions:

• What should the actual layout be? The current implementation goes with packages/<ARCH>/<LIBC>/. The other options that I can think of are
  • packages/<ARCH>-<LIBC>/
  • packages/<XBPS_ARCH>/
  • packages/<ARCH>/ with glibc and musl combined in the same dir (like the current aarch64 situation)
• This pr does not currently change the layout of subrepos relative to the main one. Should the contents of the main repo for an arch-libc combo also be moved to a main subdirectory? main, nonfree, etc would all be at the same level.
• How should the current mirrors be migrated to the new structure? Would it only be a matter of letting rsync do the job or would a one-time script be needed?

Todo

• Buildsync
  • Update to buildsync-*.nomad for new paths
    Still a bit nebulous to me how exactly would that be done.
• Mirrors
  • Nginx rules for compatibility with old paths
  • Testing the rules