Fix #2895: Problems with translations in plugins (part 1)

`ckan.lib.plugin.DefaultTranslation` relies on the extension package's
`__file__` attribute which is not available for namespace packages.

The fix is to use `ckanext.my_extension.plugin.__file__` instead. Note
that the existing code already assumes the existance of the `plugin`
module.

ckan/lib/plugins.py

@@ -537,7 +537,7 @@ def i18n_directory(self):
         i18n/
         '''
         # assume plugin is called ckanext.<myplugin>.<...>.PluginClass
-        extension_module_name = '.'.join(self.__module__.split('.')[0:2])
+        extension_module_name = '.'.join(self.__module__.split('.')[:3])
         module = sys.modules[extension_module_name]
         return os.path.join(os.path.dirname(module.__file__), 'i18n')