Extract last source package release from local VCS data

Extract last source package release from local VCS data

* lisp/emacs-lisp/package-vc.el (package-vc-archive-spec-alist):
Unmention :release-rev
(package-vc-desc->spec): Fall back on other archives if a
specification is missing.
(package-vc-main-file): Add new function, copying the behaviour of
elpa-admin.el.
(package-vc-generate-description-file): Use 'package-vc-main-file'.
(package-vc-unpack): Handle special value ':last-release'.
(package-vc-release-rev): Add new function using 'last-change'.
(package-vc-install): Pass ':last-release' as REV instead of a
release.
* lisp/vc/vc-git.el (vc-git-last-change): Add Git 'last-change'
implementation.
* lisp/vc/vc.el (vc-default-last-change): Add default 'last-change'
implementation.

This attempts to replicate the behaviour of elpa-admin.el's
"elpaa--get-last-release-commit".