Use 'package-vc-selected-packages' to store package specs

Use 'package-vc-selected-packages' to store package specs

* doc/emacs/package.texi (Fetching Package Sources): Do not promote
the usage of 'package-vc-selected-packages' to install packages.
* lisp/emacs-lisp/package-vc.el (package-vc-selected-packages): Remove
custom setter and change docstring according to these changes.
(package-vc--desc->spec): Consult 'package-vc-selected-packages' for
package specifications.
(package-vc--unpack): Add unknown package specifications to
'package-vc-selected-packages'