bug#55655: 28.1; package.el no longers respects `user-emacs-directory'

[Shitikanth]

Until emacs 27.2, package.el used to correctly pick `package-user-dir'
based on the value of `user-emacs-directory', but it now seems to ignore
it and defaults to "~/.emacs.d/elpa".

Demo:
    (setq user-emacs-directory "/tmp")
    (require 'package)
    (message "%s" package-user-dir)

Starting with "emacs -Q" this would output "~/.emacs.d/elpa".
Expected output: "/tmp/elpa" (as in emacs 27.2).

Based on git bisect, the offending commit seems to be 9973019764.