Previously we could not reliably detect whether an excuse's dependency
from a source package to a binNMU was valid, as the excuse did not
contain sufficient information to determine the set of architecture(s)
on which the dependency existed.
By modifying the representation of the dependency list in the excuse to
include an architecture list we can walk the relationships in reverse
in order to sanity-check the source -> binNMU dependency.
Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>
