Blind fix for #281

I don't understand why the bug is happening, and why this get a
FrozenImporter as it was never getting one before, but this should fix
the bug.

lib/python/pyflyby/_modules.py

@@ -7,6 +7,7 @@
 
 import ast
 from   functools                import total_ordering
+from importlib.machinery import FrozenImporter
 import itertools
 import os
 import re
@@ -250,6 +251,9 @@ def filename(self):
         # Get the filename using loader.get_filename().  Note that this does
         # more than just loader.filename: for example, it adds /__init__.py
         # for packages.
+        if isinstance(loader, FrozenImporter):
+            assert not hasattr(loader, 'get_filename')
+            return None
         filename = loader.get_filename()
         if not filename:
             return None