package.rcp: don't override package-user-dir

[npostavs]

Put what we used to override package-user-dir with as the first element
of package-directory-list instead; that way, if a user has packages
installed there, package.el can still find them.

fixes #799

---

I'm fairly sure this works, but as this change is a bit delicate, it would be good to have some more "real world" testing before merging it.

[npostavs]

I used the following code for testing, run the 2nd time with DO_NOT_CLEAN=t ./run-test-interactive.sh, and poked around in ~, package-list-packages and el-get-list-packages seemed correct.

(setq el-get-sources
      ;; smallest GNU ELPA packages I could find
      '((:name load-dir :type elpa)
        (:name num3-mode :type elpa)
        (:name memory-usage :type elpa)))

;;; install to the old package dir: ~/.emacs.d/el-get/package/elpa
;;; XXX: comment this out for second run
(el-get 'sync "load-dir" "num3-mode")

;;; now update `package-user-dir' to the standard
(push '(:name package
              :post-init
              (progn
                ;; restore package.el's original values
                (setq package-user-dir
                      (locate-user-emacs-file "elpa")

                      ;; Defaults are subdirs named "elpa" in the site-lisp dirs.
                      package-directory-list
                      (let (result)
                        (dolist (f load-path)
                          (and (stringp f)
                               (equal (file-name-nondirectory f) "site-lisp")
                               (push (expand-file-name "elpa" f) result)))
                        (nreverse result))

                      package-archives
                      '(("gnu" . "http://elpa.gnu.org/packages/")))

                ;; add package.rcp's old `package-user-dir' to
                ;; `package-directory-list', in case there are
                ;; packages installed there from before
                (let ((old-package-user-dir
                       (expand-file-name
                        (convert-standard-filename
                         (concat (file-name-as-directory
                                  default-directory)
                                 "elpa")))))
                  (when (file-directory-p old-package-user-dir)
                    (push old-package-user-dir package-directory-list)))))
      el-get-sources)

(el-get 'sync)