Do not print dependencies of external packages

This bug was hiding due to empty dependencies of external packages.
However, if the package happens to contain dependencies and be external,
it crashes to due to type mismatch.

In addition, '-L' option now prints unique external dependencies.

Author: rakhimov

graph.py

@@ -219,10 +219,10 @@ def _stabilize(node):
 
         def _print_dependencies(node):
             """Prints dependencies of the levelized components."""
-            if reduced_dependencies is None:
+            if reduced_dependencies is None or self.__is_external(node):
                 return
             for v in sorted(self.digraph[node] if reduced_dependencies
-                            else self.__dep_filter(node.dependencies()),
+                            else set(self.__dep_filter(node.dependencies())),
                             key=lambda x: (self.get_level(x), str(x))):
                 if v in self.node2cycle:
                     cycle = self.node2cycle[v]