bug#70616: 29.3; imenu in Elisp buffers omits keymap variables defined by defvar-keymap

[z@bcc32.com]

1. emacs -Q
2. Visit some Elisp file that uses defvar-keymap, e.g., C-x C-f ~/src/emacs/trunk/lisp/button.el
3. M-x imenu

button-map is defined in this file, using the defvar-keymap macro, but
it doesn't show up in the imenu index.  Other things defined in this
file do show up.  I think this is just a matter of updating
lisp-imenu-generic-expression, but I'm not familiar enough to say for
certain.

In GNU Emacs 29.3 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
version 1.18.0, Xaw3d scroll bars)
Windowing system distributor 'HC-Consult', version 11.0.12014000
System Description: NixOS 23.11 (Tapir)

Configured using:
 'configure
 --prefix=/nix/store/2cgdvmzrgkihvjx4w34sjj29zjr2dy8b-emacs-29.3
 --disable-build-details --with-modules --with-x-toolkit=lucid
 --with-xft --with-cairo --with-compress-install
 --with-toolkit-scroll-bars --with-native-compilation
 --without-imagemagick --without-small-ja-dic --with-tree-sitter
 --with-xinput2 --without-xwidgets --with-dbus --with-selinux'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP
NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XAW3D XDBE XIM XINPUT2 XPM
LUCID ZLIB

Important settings:
  value of $EMACSLOADPATH: 
  value of $EMACSNATIVELOADPATH: 
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Org