* lisp/emacs-lisp/package.el: New "external" package status

* lisp/emacs-lisp/package.el: New "external" package status

An external package is any installed package that's not built-in
and not from `package-user-dir', which usually means it's from an
entry in `package-directory-list'.  They are treated much like
built-in packages, in that they cannot be through the Package Menu
deleted and are not considered for upgrades.

(package-desc-status): Identify if a package is installed outside
`package-user-dir'.
(package-menu--print-info-simple)
(package-menu--status-predicate): Add support for it.

* etc/NEWS: Document it.