elisp: Set 'guix-directory' when needed

Before the previous commit (6476f612aa),
'guix-directory' variable was blindly set to the latest "guix pull"
directory.  But it is not good because a user may not have that pulled
directory.  It is better to ask Guile where guix modules are placed and
just use this directory.

* elisp/guix-repl.el (guix-directory): New function that sets
'guix-directory' variable if needed.
(guix-read-directory): Use 'guix-directory' function instead of variable.
* elisp/guix-location.el (guix-find-location): Likewise.
* elisp/guix-license.el (guix-license-file): Likewise.
* doc/emacs-guix.texi (More Package Commands): Remove documentation of
'guix-directory'.

doc/emacs-guix.texi

@@ -501,24 +501,6 @@ complete a package name.
 
 @end table
 
-@vindex guix-directory
-If you are contributing to Guix, you may find it useful for
-@kbd{M-x@tie{}guix-find-package-location-file} and
-@kbd{M-x@tie{}guix-find-package-definition} to open locations from
-your Git checkout.  This can be done by setting @code{guix-directory}
-variable.  For example, after this:
-
-@example
-(setq guix-directory "~/src/guix")
-@end example
-
-@kbd{M-x guix-edit guix} opens
-@file{~/src/guix/gnu/packages/package-management.scm} file.
-
-Also you can use @kbd{C-u} prefix argument to specify a directory just
-for the current @kbd{M-x@tie{}guix-find-package-location-file} or
-@kbd{M-x@tie{}guix-edit} command.
-
 @vindex guix-read-package-name-function
 By default, when you should specify a package name (for example, after
 @kbd{M-x guix-edit}), you will be prompted for it in the minibuffer,

elisp/guix-license.el

@@ -31,7 +31,7 @@
   "Return name of the file with license definitions.
 DIRECTORY is a directory with Guix source (`guix-directory' by default)."
   (expand-file-name "guix/licenses.scm"
-                    (or directory guix-directory)))
+                    (or directory (guix-directory))))
 
 (defun guix-lookup-license-url (license)
   "Return URL of a LICENSE."

elisp/guix-location.el

@@ -64,7 +64,7 @@ DIRECTORY (if it is specified and exists)."
   (cl-multiple-value-bind (file line column)
       (split-string location ":")
     (let* ((file-name (expand-file-name file (or directory
-                                                 guix-directory)))
+                                                 (guix-directory))))
            (file-name (if (file-exists-p file-name)
                           file-name
                         (guix-eval-read

elisp/guix-repl.el

@@ -434,18 +434,30 @@ additional internal REPL if it exists."
 
 (defvar guix-directory nil
   "Default directory with Guix source.
-If it is not set by a user, it is set after starting Guix REPL.
+
 This directory is used to find packages and licenses by such
 commands as `guix-find-package-definition' or
-`guix-find-license-definition'.")
+`guix-find-license-definition'.
+
+Most likely you don't need to set this variable because it is set
+automatically when needed.  However, you can still set it if you
+really want; your value will not be overwritten.")
+
+(defun guix-directory ()
+  "Set if needed and return `guix-directory'."
+  (or guix-directory
+      (let* ((guix.scm (guix-eval-read "(%search-load-path \"guix\")"))
+             (dir (and guix.scm
+                       (file-name-directory guix.scm))))
+        (setq guix-directory dir))))
 
 (defun guix-read-directory ()
   "Return `guix-directory' or prompt for it.
 This function is intended for using in `interactive' forms."
   (if current-prefix-arg
       (read-directory-name "Directory with Guix modules: "
-                           guix-directory)
-    guix-directory))
+                           (guix-directory))
+    (guix-directory)))
 
 ;; XXX Remove `guix-latest-directory' in future: it exists for backward
 ;; compatibility (in the past "guix pull" populated