fw/plugin: do not try to load imported classes

When scanning modules for plugins, only consider classes defined in that
module, but not those imported into it. This will prevent log output
being polluted with repeated "Skipping..." messages for imported plugin
bases.

wa/framework/plugin.py

@@ -631,6 +631,8 @@ class PluginLoader(object):
                 if inspect.isclass(obj):
                     if not issubclass(obj, Plugin):
                         continue
+                    if obj.__module__ != module.__name__:
+                        continue
                     if not obj.kind:
                         message = 'Skipping plugin {} as it does not define a kind'
                         self.logger.debug(message.format(obj.__name__))