Reject some excuses with unsatisfiable depends

Ideally we would reject all items with known unsatisfiable
dependencies as they would not be installable in testing.  However,
there are a few known corner cases where we still want to migrate them
(notably when they are already broken in testing).

This commit is an attempt to weed out some of the "obviously" broken
items that will not successfully migrate.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -947,7 +947,8 @@ class Britney(object):
         # analyze the dependency fields (if present)
         deps = binary_u.depends
         if not deps:
-            return
+            return True
+        is_all_ok = True
 
         # for every dependency block (formed as conjunction of disjunction)
         for block, block_txt in zip(parse_depends(deps, False), deps.split(',')):
@@ -972,6 +973,8 @@ class Britney(object):
             if not packages:
                 excuse.addhtml("%s/%s unsatisfiable Depends: %s" % (pkg, arch, block_txt.strip()))
                 excuse.addreason("depends")
+                if arch not in self.options.break_arches:
+                    is_all_ok = False
                 continue
 
             # for the solving packages, update the excuse to add the dependencies
@@ -983,6 +986,7 @@ class Britney(object):
                         excuse.add_dep(p, arch)
                 else:
                     excuse.add_break_dep(p, arch)
+        return is_all_ok
 
     # Package analysis methods
     # ------------------------
@@ -1409,7 +1413,9 @@ class Britney(object):
                 # if the package is architecture-dependent or the current arch is `nobreakall'
                 # find unsatisfied dependencies for the binary package
                 if binary_u.architecture != 'all' or arch in self.options.nobreakall_arches:
-                    self.excuse_unsat_deps(pkg, src, arch, suite, excuse)
+                    is_valid = self.excuse_unsat_deps(pkg, src, arch, suite, excuse)
+                    if not is_valid and not source_t:
+                        update_candidate = False
 
             # if there are out-of-date packages, warn about them in the excuse and set update_candidate
             # to False to block the update; if the architecture where the package is out-of-date is