Don't try to build dependency trees for non-existent packages

get_full_tree may be passed an initial package which does not exist in the
target suite, for example because a package declares an obsolete conflict
on a second package which has subsequently been removed.

britney.py

@@ -2132,6 +2132,7 @@ class Britney:
         """
         packages = [pkg]
         binaries = self.binaries[suite][arch][0]
+        if pkg in binaries:
             l = n = 0
             while len(packages) > l:
                 l = len(packages)
@@ -2139,6 +2140,8 @@ class Britney:
                     packages.extend([x for x in binaries[p][RDEPENDS] if x not in packages and x in binaries])
                 n = l
             return packages
+        else:
+            return []
 
     def iter_packages(self, packages, selected, hint=False, nuninst=None):
         """Iter on the list of actions and apply them one-by-one