find-alternate-file: void dired-directory

[Markus Rost]

The recent change

        * files.el (find-alternate-file): Handle dired-directory
        like buffer-file-name.

causes a problem:  Call

C-x C-f .emacs          [find-file .emacs]
C-x C-v C-k (return)    [Try to find alternate file "~/"] 

You get an error:

call-interactively: Symbol's value as variable is void: dired-directory

This happens when dired.el is not loaded, since then dired-directory
is not defined.

I am not sure how to fix this.  Defvar dired-directory in files.el?
Autoload dired-directory?  Handle arg FILENAME of find-alternate-file
specially if it is a directory?