Package archives now have priorities.

Package archives now have priorities.

* lisp/package.el: Provide repository priorities.
(package-archive-priorities): New variable.
(package--add-to-alist): New function.
(package--add-to-archive-contents): Use it.
(package-menu--find-upgrades): Use it as well. Small clean up to
make the use of the package name here explicit.
(package-archive-priority): New function.
(package-desc-priority-version): New function.

Fixes: debbugs:19296