Extract package-fetch and related functionality

Extract package-fetch and related functionality

Note that the "package kind" was renamed from "source" to "vc".

* package-vc.el: (package-vc-commit): Copy from package.el
(package-vc-version): Add new function
(package-vc-generate-description-file): Add new function.
(package-vc-unpack): Add new function.
(package-vc-fetch): Copy from package.el
(package-checkout): Add alias for package-vc-fetch
* package.el (package-devel-dir): Remove option.  The checkouts are
stored in package-user-dir
(package-desc): Handle (vc . VERS) version strings
(package-desc-full-name): Return the plain name for vc packages
(package-devel-commit): Move function to package-vc
(package-load-descriptor): Refactor according to other changes
(package-load-all-descriptors): Remove package-devel-dir
(package-unpack): Remove vc package handling
(package-generate-description-file): Remove special handling for vc
packages
(package-install-from-archive): Remove special handling for vc
packages
(package-fetch): Move function to package-vc
(package-desc-status): Use "vc" instead of "source"
(package--remove-hidden): Use "vc" instead of "source"
(package-menu--print-info-simple): Refactor according to other changes