Use relative imports for the gdbus/codegen package

But not yet for the config module (but is taken care
of in a followup patch).

https://bugzilla.gnome.org/show_bug.cgi?id=650763

gio/gdbus-2.0/codegen/codegen_docbook.py

@@ -25,9 +25,9 @@ import sys
 import re
 
 import config
-import utils
-import dbustypes
-import parser
+from . import utils
+from . import dbustypes
+from . import parser
 
 # ----------------------------------------------------------------------------------------------------