[commands] Add BotBase.get_cog_commands to get all a cog's commands.

Self-explanatory. This should help create help commands for a cog
more easily.

discord/ext/commands/bot.py

@@ -484,6 +484,31 @@ class BotBase(GroupMixin):
         """
         return self.cogs.get(name)
 
+    def get_cog_commands(self, name):
+        """Gets a unique set of the cog's registered commands
+        without aliases.
+
+        If the cog is not found, an empty set is returned.
+
+        Parameters
+        ------------
+        name: str
+            The name of the cog whose commands you are requesting.
+
+        Returns
+        ---------
+        Set[:class:`Command`]
+            A unique set of commands without aliases that belong
+            to the cog.
+        """
+
+        try:
+            cog = self.cogs[name]
+        except KeyError:
+            return set()
+
+        return {c for c in self.all_commands.values() if c.instance is cog}
+
     def remove_cog(self, name):
         """Removes a cog from the bot.