Reserves command names (#2977)

* Reserves command names

  - Currently, only reserving ``cancel``
  - This should only impact matching command qualified names
  - This also checks aliases
  - This makes cc and alias use the new module constant with info about
  this
  - Module constant is available for use by 3rd party cogs which may
  dynamically create responses.

* Change misleading var name

* style

* Thanks Flame!

* Handles issues with CC

redbot/cogs/customcom/customcom.py

@@ -222,6 +222,9 @@ class CustomCommands(commands.Cog):
 
         Note: This command is interactive.
         """
+        if command in (*self.bot.all_commands, *commands.RESERVED_COMMAND_NAMES):
+            await ctx.send(_("There already exists a bot command with the same name."))
+            return
         responses = await self.commandobj.get_responses(ctx=ctx)
         try:
             await self.commandobj.create(ctx=ctx, command=command, response=responses)
@@ -241,7 +244,7 @@ class CustomCommands(commands.Cog):
         Example:
         - `[p]customcom create simple yourcommand Text you want`
         """
-        if command in self.bot.all_commands:
+        if command in (*self.bot.all_commands, *commands.RESERVED_COMMAND_NAMES):
             await ctx.send(_("There already exists a bot command with the same name."))
             return
         try: