support mixing noarch and archful packages in a yum repo build
Currently the YumRepoBuild class assumes that *every* package you pass
to it was built for the requested architecture. But this makes it
impossible to put noarch packages into the repository, because
YumRepoBuild will look for an arch-specific package when only a noarch
package was produced.
Change YumRepoBuild to only copy over packages if they actually produced
the requested arch, otherwise skip them.