base: Make the Python module loader PEP302 compliant

The custom Python loader didn't comply with PEP302 for two reasons:

 * Previously, we would overwrite old modules on name
   conflicts. PEP302 explicitly states that: "If there is an existing
   module object named 'fullname' in sys.modules, the loader must use
   that existing module".

 * The "__package__" attribute wasn't set. PEP302: "The __package__
   attribute must be set."

This changeset addresses both of these issues.

src/python/importer.py

@@ -54,6 +54,10 @@ class CodeImporter(object):
         import imp
         import os
         import sys
+
+        try:
+            mod = sys.modules[fullname]
+        except KeyError:
             mod = imp.new_module(fullname)
             sys.modules[fullname] = mod
 
@@ -68,6 +72,9 @@ class CodeImporter(object):
 
             if os.path.basename(srcfile) == '__init__.py':
                 mod.__path__ = fullname.split('.')
+                mod.__package__ = fullname
+            else:
+                mod.__package__ = fullname.rpartition('.')[0]
             mod.__file__ = srcfile
 
             exec code in mod.__dict__