Usability improvements to commands in package-x.el.
* lisp/emacs-lisp/package-x.el (package-archive-upload-base): Make it a
defcustom.
(package--update-file): Doc fix. Accept relative file names.
(package--archive-contents-from-file): Remove the argument, since
it's necessarily always "archive-contents".
(package-maint-add-news-item): Pass relative file name args to
package--update-file.
(package-upload-buffer-internal): Prompt for a destination if
package-archive-upload-base is invalid. Create the directory if
it does not exist.
(package-upload-buffer, package-upload-file): Doc fix.