When a dependency cannot be found in source or binary packages of the
selected architecture, then don't look for binary packages in other
architectures package indexes.
This leads to a confusing error down the line when building packages for
extra_repos_systemd (and e.g. stable branches), because we don't have
binary packages for all arches that pmbootstrap supports, and so it
tries to fetch a non-existing APKINDEX.
Instead, print a nice error about a non-existing dependency.
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