Use 'elpa-packages' files for VC metadata

Use 'elpa-packages' files for VC metadata

* lisp/emacs-lisp/package-vc.el (package-vc-default-backend): Add new
option.
(package-vc-archive-spec-alist): Add new variable to store the
contents of 'elpa-packages' for each archive.
(pacakge-vc-desc->spec): Add function to query package specifications.
(package-vc--read-archive-data): Add a 'package-read-archive-hook'
implementation.
(package-vc--download-and-read-archives): Add a
'package-refresh-contents-hook' implementation.
(package-vc-main-file): Remove function.
(package-vc-generate-description-file): Use package specifications.
(package-vc-unpack-1): Adapt to previous changes.
(package-vc-unpack): Adapt to previous changes.
(package-vc-sourced-packages-list): Adapt to previous changes.
(package-vc-install): Adapt to previous changes.
* lisp/emacs-lisp/package.el (package-read-archive-hook): Allow
extending 'package-read-all-archive-contents' using a hook.
(package-read-all-archive-contents): Use 'package-read-archive-hook'.
(package-refresh-contents-hook): Allow extending
'package-refresh-contents' using a hook.
(package-refresh-contents): Use 'package-refresh-contents-hook'.