Get long package description for installed packages from installed files

* doc/lispref/package.texi (Archive Web Server): New; document web
server interface.

* lisp/emacs-lisp/package.el (package--get-description): New; get long
description from installed files.
(describe-package-1): Use it, improve comments. No longer writing
NAME-readme.txt.

* test/lisp/emacs-lisp/package-tests.el:
(package-test-describe-package): There is now a description for an
installed package.
(package-test-describe-installed-multi-file-package): New test.

test/lisp/emacs-lisp/package-tests.el

@@ -435,11 +435,24 @@ Must called from within a `tar-mode' buffer."
      (save-excursion (should (search-forward "Summary: A single-file package with no dependencies" nil t)))
      (save-excursion (should (search-forward "Homepage: http://doodles.au" nil t)))
      (save-excursion (should (re-search-forward "Keywords: \\[?frobnicate\\]?" nil t)))
-     ;; No description, though. Because at this point we don't know
-     ;; what archive the package originated from, and we don't have
-     ;; its readme file saved.
+     (save-excursion (should (search-forward "This package provides a minor mode to frobnicate"
+                                             nil t)))
      )))
 
+(ert-deftest package-test-describe-installed-multi-file-package ()
+  "Test displaying of the readme for installed multi-file package."
+
+  (with-package-test ()
+    (package-initialize)
+    (package-refresh-contents)
+    (package-install 'multi-file)
+    (with-fake-help-buffer
+     (describe-package 'multi-file)
+     (goto-char (point-min))
+     (should (search-forward "Homepage: http://puddles.li" nil t))
+     (should (search-forward "This is a bare-bones readme file for the multi-file"
+                             nil t)))))
+
 (ert-deftest package-test-describe-non-installed-package ()
   "Test displaying of the readme for non-installed package."