Improve reproducibility of generated -pkg.el files

* lisp/emacs-lisp/package.el (package-generate-description-file):
Don't include the full name of the source file in the header,
since that varies non-reproducibly according to the build directory.
https://bugs.debian.org/972861
Note that elpa.gnu.org's admin/archive-contents.el does this by hand
and already only includes the nondirectory part.

lisp/emacs-lisp/package.el

@@ -963,7 +963,8 @@ untar into a directory named DIR; otherwise, signal an error."
       (write-region
        (concat
         ";;; Generated package description from "
-        (replace-regexp-in-string "-pkg\\.el\\'" ".el" pkg-file)
+        (replace-regexp-in-string "-pkg\\.el\\'" ".el"
+                                  (file-name-nondirectory pkg-file))
         "  -*- no-byte-compile: t -*-\n"
         (prin1-to-string
          (nconc