Avoid marking some valid excuses as impossible.

When considering an excuse for pkg1/arch, a dependency on either of
pkg2/source or pkg2/arch should be considered acceptable so long
as there is a corresponding excuse.

Dependencies from pkg1/source to pkg2/arch will still be considered
"impossible", as pkg1's excuse does not contain any information
regarding the architecture(s) on which its dependency to pkg2 exists.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -1596,8 +1596,20 @@ class Britney:
 
         # invalidate impossible excuses
         for e in self.excuses:
+            # parts[0] == package name
+            # parts[1] == optional architecture
+            parts = e.name.split('/')
             for d in e.deps:
-                if d not in upgrade_me and d not in unconsidered:
+                ok = False
+                if d in upgrade_me or d in unconsidered:
+                    ok = True
+                # if the excuse is for a binNMU, also consider d/$arch as a
+                # valid excuse
+                if len(parts) == 2:
+                    bd = '%s/%s' % (d, parts[1])
+                    if bd in upgrade_me or bd in unconsidered:
+                        ok = True
+                if not ok:
                     e.addhtml("Impossible dependency: %s -> %s" % (e.name, d))
         self.invalidate_excuses(upgrade_me, unconsidered)