Allow specifying local ELPA mirrors in package-archives.
* emacs-lisp/package.el (package-archives): Accept either ordinary
directory names, in addition to HTTP URLs.
(package--with-work-buffer): New macro. Handle normal directories.
(package-handle-response): Don't display the failing buffer.
(package-download-single, package-download-tar)
(package--download-one-archive): Use package--with-work-buffer.
(package-archive-base): Rename from package-archive-url.