Based on experimentation, the canonical module name needs to be in sys.modules...

Based on experimentation, the canonical module name needs to be in sys.modules on Python prior to 3.3, but must be omitted on Python 3.3 and later.

--HG--
branch : feature/issue-229

pkg_resources/extern/__init__.py

@@ -29,7 +29,10 @@ class VendorImporter:
         for prefix in self.search_path:
             try:
                 __import__(prefix + target)
-                mod = sys.modules[fullname] = sys.modules.pop(prefix + target)
+                mod = sys.modules[prefix + target]
+                sys.modules[fullname] = mod
+                if sys.version_info > (3, 3):
+                    del sys.modules[prefix + target]
                 return mod
             except ImportError:
                 pass