Hybrid mirror: support both flat and $repo/os/$arch layouts

CI: packages go into repo/antergos-pkgs/os/x86_64/ (standard layout),
then flat copies are made at repo/ root for backward compat.
Mirrorlist now has both Server lines, tried in order by pacman.

packages/antergos-next-mirrorlist/antergos-next-mirrorlist

@@ -2,4 +2,7 @@
 ## Generated by antergos-pkgs CI
 ## Use pacman-mirrors or rankmirrors to select fastest mirror
 
+# Flat layout (packages at repo root)
 Server = https://antergos-next.github.io/antergos-pkgs
+# Standard layout (packages in $repo/os/$arch/)
+Server = https://antergos-next.github.io/antergos-pkgs/$repo/os/$arch