Define missing groups for ERC modules

* lisp/erc/erc-imenu.el (erc-imenu): Define group even though there
aren't any associated faces or user options because `imenu' is a
global module, and `define-erc-module' calls `define-minor-mode',
which needs a group for its mode variable.  This should have been
handled by bug#60954 but wasn't.
* lisp/erc/erc-sound.el (erc-sound): Move group definition above
module definition.
* lisp/erc/erc-spelling.el (erc-spelling): Define group.

lisp/erc/erc-imenu.el

@@ -41,6 +41,10 @@
 (require 'erc)
 (require 'imenu)
 
+(defgroup erc-imenu nil
+  "Imenu integration for ERC."
+  :group 'erc)
+
 (defun erc-unfill-notice ()
   "Return text from point to a computed end as a string unfilled.
 Don't rely on this function, read it first!"

lisp/erc/erc-sound.el

@@ -47,6 +47,10 @@
 
 (require 'erc)
 
+(defgroup erc-sound nil
+  "Make ERC play bells and whistles while chatting with people."
+  :group 'erc)
+
 ;;;###autoload(put 'ctcp-sound 'erc--module 'sound)
 ;;;###autoload(autoload 'erc-sound-mode "erc-sound")
 (define-erc-module sound ctcp-sound
@@ -61,10 +65,6 @@ and play sound files as requested."
 
 (erc-define-catalog-entry 'english 'CTCP-SOUND "%n (%u@%h) plays %s:%m")
 
-(defgroup erc-sound nil
-  "Make ERC play bells and whistles while chatting with people."
-  :group 'erc)
-
 (defcustom erc-play-sound t
   "Play sounds when you receive CTCP SOUND requests."
   :type 'boolean)

lisp/erc/erc-spelling.el

@@ -33,6 +33,10 @@
 (require 'erc)
 (require 'flyspell)
 
+(defgroup erc-spelling nil
+  "Flyspell integration for ERC."
+  :group 'erc)
+
 ;;;###autoload(autoload 'erc-spelling-mode "erc-spelling" nil t)
 (define-erc-module spelling nil
   "Enable flyspell mode in ERC buffers."