Support filtering by keywords in package listings.

Support filtering by keywords in package listings.

* emacs-lisp/package.el (package-built-in-p): Support both
built-in and the package.el converted package descriptions.
(package-show-package-list): Allow keywords.
(package-keyword-button-action): Use it instead of
`finder-list-matches'.
(package-menu-filter-interactive): Interactive filtering (by
keyword) function.
(package-menu--generate): Support keywords and change keymappings
and headers when they are given.
(package--has-keyword-p): Helper function.
(package-menu--refresh): Use it.
(package--mapc): Helper function.
(package-all-keywords): Use it.
(package-menu-mode-map): Set up menu items and keybindings to
provide a filtering UI.