e.g. fedora has separate repo with primary and secondary architectures. When these are used in one buildtag, mergerepo will fail on missing buildarchs in these repos. We could create additional field in external repo table, specyfing for which archs is this repo valid. It probably should be done on level of tag-extrepo table, so some tags can restrict architecture expansion. If there is no architecture specified, all buildarchs are expected to be in external repo - which is same as current behaviour.