[commands] unload cog submodules

When unloading cogs, currently we do not remove submodules from
sys.modules, meaning they will not be reloaded. Removing here
makes new imports reload from file. Of course, any already imported
modules will still hold a reference to the old module, since they
will not re-import it, and will not be forcably unloaded.

discord/ext/commands/bot.py

@@ -768,6 +768,9 @@ class BotBase(GroupMixin):
             del lib
             del self.extensions[name]
             del sys.modules[name]
+            for module in list(sys.modules.keys()):
+                if _is_submodule(lib_name, module):
+                    del sys.modules[module]
 
     # command processing