Separate built-in packages from elpa packages, for efficiency.

Separate built-in packages from elpa packages, for efficiency.

* emacs-lisp/package.el: Don't put built-in packages in
package-alist, to avoid loading inefficiencies.
(package-built-in-p): Make VERSION optional, and treat it as a
minimum acceptable version.
(package-activate): Search separately for built-in packages.  Emit
a warning if a dependency fails.
(define-package): Handle most common case, where there is no
obsolete package, first.
(package-compute-transaction): Print required version in error.
(package--initialized): New variable.
(list-packages): Use it.
(package-initialize): Optional arg NO-ACTIVATE.  Don't put
built-in packages in packages-alist; keep it separate.  Set
package--initialized.
(describe-package): Avoid activating packages as a side-effect.
Search separately for built-in packages.
(describe-package-1): Handle the case where an elpa package is
simultaneously built-in and available/installed.
(package-installed-p, package--generate-package-list): Search
separately for built-in packages.
(package-load-descriptor): Doc fix.