Load descriptors for installed Emacs Lisp packages.
This looks for package subdirectories in `package-user-dir' and
`package-directory-list'. The variable `package-load-list'
controls which package subdirectories may be loaded.
In each valid package subdirectory, this function loads the
description file containing a call to `define-package', which
updates `package-alist' and `package-obsolete-alist'.