Mark 'package-vc-update' as interactive

Mark 'package-vc-update' as interactive

* lisp/emacs-lisp/package-vc.el (package-vc--sourced-packages-list):
Remove function in favour of 'package-vc--read-package-name'.
(package-vc--read-package-name):
Extract out common functionality.
(package-vc--read-package-desc): Add auxiliary function based on
'package-vc--read-package-name'.
(package-vc-update): Add interactive spec using
'package-vc--read-package-desc'.
(package-vc-install): Use 'package-vc--read-package-desc'.
(package-vc-checkout): Use 'package-vc--read-package-desc'.
(package-vc--read-pkg): Remove in favour of 'package-vc--read-package-desc'.
(package-vc-refresh): Use 'package-vc--read-package-desc'.
(package-vc-prepare-patch): Use 'package-vc--read-package-desc'.