Prepare package.el to be published on GNU ELPA

Prepare package.el to be published on GNU ELPA

* lisp/emacs-lisp/package.el: Add Compat as a dependency.
(package--native-compile-async): Check if 'native-comp-available-p' is
bound.
(lm-homepage, lm-website): Use backwards-compatible alias
'lm-homepage'.
(package-buffer-info): Call 'lm-maintainer' if 'lm-maintainers' is not
defined.
(describe-package-1): Avoid using 'make-separator-line' if not bound.
(package-report-bug): Expand 'custom--standard-value' definition.

For the background and motivation behind these changes, please consult
this thread:
https://lists.gnu.org/archive/html/emacs-devel/2023-03/msg00995.html.