Convert ERC's Imenu integration into proper module

Convert ERC's Imenu integration into proper module

* lisp/erc/erc-goodies.el: Don't add Imenu hooks to `erc-mode-hook' at
top level.  Remove autoload for `erc-create-imenu-index' because it
already exists in the `erc-imenu' library.
(erc-imenu-setup): Move to the erc-imenu library.
* lisp/erc/erc-imenu.el (erc-unfill-notice): Allow modifications to
read-only text.  Thanks to Yusef Aslam for reporting this bug.
(erc-imenu-setup): Move here from goodies.
(erc-imenu--create-index-function): New helper var to hold previous
local value of `imenu-create-index-function'.  Perhaps advice should
be used instead, but a cursory search of the Emacs code base reveals
no such usage.
(erc-imenu-mode, erc-imenu-enable, erc-imenu-disable): Create "new"
ERC module for Imenu integration.
* lisp/erc/erc.el (erc-modules): Add `imenu' to default value and
create widget menu item.  Update package-version.
* test/lisp/erc/erc-tests.el (erc-tests--modules): Add
`imenu'.  (Bug#60954)