Extract variable for extant name. Add comment about the hack.

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

pkg_resources/extern/__init__.py

@@ -38,11 +38,17 @@ class VendorImporter:
         root, base, target = fullname.partition(self.root_name + '.')
         for prefix in self.search_path:
             try:
-                __import__(prefix + target)
-                mod = sys.modules[prefix + target]
+                extant = prefix + target
+                __import__(extant)
+                mod = sys.modules[extant]
                 sys.modules[fullname] = mod
+                # mysterious hack:
+                # Remove the reference to the extant package/module
+                # on later Python versions to cause relative imports
+                # in the vendor package to resolve the same modules
+                # as those going through this importer.
                 if sys.version_info > (3, 3):
-                    del sys.modules[prefix + target]
+                    del sys.modules[extant]
                 return mod
             except ImportError:
                 pass