utils: Add a %guix-source-root-directory procedure.

* guix/utils.scm (%guix-source-root-directory): New procedure.

guix/utils.scm

@@ -78,6 +78,7 @@
             substitute-keyword-arguments
             ensure-keyword-arguments
 
+            %guix-source-root-directory
             current-source-directory
 
             nix-system->gnu-triplet
@@ -1031,6 +1032,10 @@ environment variable name like \"XDG_CONFIG_HOME\"; SUFFIX is a suffix like
 ;;; Source location.
 ;;;
 
+(define (%guix-source-root-directory)
+  "Return the source root directory of the Guix found in %load-path."
+  (dirname (absolute-dirname "guix/packages.scm")))
+
 (define absolute-dirname
   ;; Memoize to avoid repeated 'stat' storms from 'search-path'.
   (mlambda (file)