caches need to be namespaced

[miyagawa]

currently both build artifacts and tarballs are shared under the same Carmel repo, but this should better be namespaced under CPAN/DarkPAN urls. it's unrealistic for identical PAUSE ID files to be different across mirrors (even for DarkPANs) but i think it's cleaner if they're separated anyway. Probably some normalization could be done so that http/https and known CPAN mirrors are considered the same, etc

see also #60