* lisp/emacs-lisp/package-x.el (package-upload-buffer-internal): Pass

* lisp/emacs-lisp/package-x.el (package-upload-buffer-internal): Pass
`extras' slot from `package-desc' to `package-make-ac-desc'.

* lisp/emacs-lisp/package.el (package-desc-from-define): Accept
additional arguments as plist, convert it to an alist and store it
in the `extras' slot.
(package-generate-description-file): Convert extras alist back to
plist and append to the `define-package' form arguments.
(package--alist-to-plist): New function.
(package--ac-desc): Add `extras' slot.
(package--add-to-archive-contents): Check if the archive-contents
vector is long enough, and if it is, pass its `extras' slot value
to `package-desc-create'.
(package-buffer-info): Call `lm-homepage', pass the returned value
to `package-desc-from-define'.
(describe-package-1): Render the homepage button.

* test/automated/package-test.el (simple-single-desc-1-4): Remove, it
was unused.
(simple-single-desc): Expect :homepage property.
(multi-file-desc): Same.
(with-package-test): Do not save previous `default-directory'
value, let-bind the var instead.
(package-test-install-single): Expect :homepage property in the
generated pkg file.
(package-test-describe-package): Expect Homepage button.
(package-test-describe-non-installed-package)
(package-test-describe-non-installed-multi-file-package): Same.
(package-test-describe-not-installed-package): Remove, it was a
duplicate.

* test/automated/package-x-test.el
(package-x-test--single-archive-entry-1-3): Expect :homepage
property.
(package-x-test--single-archive-entry-1-4): Expect nil extras slot.

* test/automated/data/package/archive-contents: Add :homepage
properties to `simple-single' and `multi-file'.

* test/automated/data/package/simple-single-1.3.el: Add URL header.

Fixes: debbugs:13291