Reformat package.el message strings for future l10n

Reformat package.el message strings for future l10n

* lisp/emacs-lisp/package.el (package-buffer-info)
(package--download-one-archive, package-install-selected-packages)
(package-autoremove, describe-package-1, package-menu-toggle-hiding)
(package-menu-hide-package, package-menu--mark-upgrades-1)
(package-menu--list-to-prompt, package-menu--prompt-transaction-p)
(package-menu-execute, package-menu--find-and-notify-upgrades):
Reformat message strings to remove need for plural computation.  Try
to put full sentences in source string literals.
* test/lisp/emacs-lisp/package-tests.el
(package-test-describe-package): Update to match new message format.