Fix infinite recursion in PrebuiltsReady when graph is cyclic.

parallel_emerge

Index: parallel_emerge
diff --git a/parallel_emerge b/parallel_emerge
index 5b09ec6e428f8384160fe9624221555f5b30edc6..971e6a30b334dc1b211ea7a218f704e9e5eeb394 100755
--- a/parallel_emerge
+++ b/parallel_emerge
@@ -819,11 +819,12 @@ class DepGraphGenerator(object):
         if pkg not in pkg_db:
           cache[pkg] = False
         else:
+          cache[pkg] = True
           for dep in deps_map[pkg]["needs"]:
             if not PrebuiltsReady(dep, pkg_db, cache):
               cache[pkg] = False
               break
-        return cache.setdefault(pkg, True)
+        return cache[pkg]
 
       def LastModifiedWithDeps(pkg, pkg_db, cache):
         """Calculate the last modified time of a package and its dependencies.