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/core/commands/commands.py

@@ -28,8 +28,14 @@ __all__ = [
     "GroupMixin",
     "command",
     "group",
+    "RESERVED_COMMAND_NAMES",
 ]
 
+#: The following names are reserved for various reasons
+RESERVED_COMMAND_NAMES = (
+    "cancel",  # reserved due to use in ``redbot.core.utils.MessagePredicate``
+)
+
 _ = Translator("commands.commands", __file__)
 
 
@@ -155,6 +161,12 @@ class Command(CogCommandMixin, commands.Command):
         super().__init__(*args, **kwargs)
         self._help_override = kwargs.pop("help_override", None)
         self.translator = kwargs.pop("i18n", None)
+        if self.parent is None:
+            for name in (self.name, *self.aliases):
+                if name in RESERVED_COMMAND_NAMES:
+                    raise RuntimeError(
+                        f"The name `{name}` cannot be set as a command name. It is reserved for internal use."
+                    )
 
     def _ensure_assignment_on_copy(self, other):
         super()._ensure_assignment_on_copy(other)