discovery: Handle edge case in scheme-files when looking at symlinks.

Previously, this code would cause crashes in Guix (running guix package -s for
example) which could be experienced when Emacs creates temporary files in the
gnu/packages/patches directory when a patch file has been edited, but not
saved.

* guix/discovery.scm (scheme-files): Add else clause to cond used when
handling symlinks.

guix/discovery.scm

@@ -78,7 +78,9 @@ DIRECTORY is not accessible."
                                  ((= stat:type 'directory)
                                   (append (scheme-files absolute)
                                           result))
-                                 (_ result)))))
+                                 (_ result)))
+                              (else
+                               result)))
                        (else
                         result))))))
               '()