This causes Multi-arch dependencies like "pkg:i386" to show up as
unsatisfiable in excuses.
Previously, the dependency would be checked on the wrong architecture
(if available) and cause the package to become a valid candidate. The
package would still be prevent from migrating as the installability
checker does not know of the "pkg:i386" package.
Signed-off-by: Niels Thykier <niels@thykier.net>