related to: void-linux/void-infrastructure#142

@the-maldridge expressed interest in the new layout being .../<XBPS_ARCH>/..., and I think I agree that this would make the most sense.

I think the best way to implement this in xbps (and having the side benefit of preventing needing to -R other arch's repos when installing something for another XBPS_ARCH) would be to allow templating repo urls:

repository=https://repo-default.voidlinux.org/current/{{XBPS_ARCH}}/

1. What delimiter characters should be used to template this? {{XBPS_ARCH}}, %XBPS_ARCH% were thoughts I had.
2. should other templated values be allowed, like ARCH and LIBC?
3. Could this also be an opportunity to change <arch> -> <arch>-glibc? <arch> would have to stick around as a legacy alias of course.

I am interested in implementing this once some of the details are hashed out