services: xorg-wrapper: Support xorg server input rewriting.

This patch adds support for proper xorg server input rewriting. It uses the
lookup-package-direct-input procedure to configure the X server paths
dynamically, instead of always using the hard coded package. Something like
this is now possible:

(define other-mesa
  (package-input-rewriting/spec `(("mesa" . ,(const other-mesa)))))

(xorg-configuration
 (xorg-configuration
  (server (other-mesa xorg-server))))

Without this patch the X server would still be configured with mesa (causing
version issues), and not with other-mesa (as per the input rewrite).

* gnu/services/xorg.scm (xorg-configuration-server-package-path)
(xorg-configuration-dri-driver-path, xorg-configuration-xkb-bin-dir)
(xorg-configuration-xkb-dir): New procedures.
(xorg-wrapper): Use them for dri and xkb paths.

Signed-off-by: 宋文武 <iyzsong@member.fsf.org>

gnu/services/xorg.scm

@@ -358,6 +358,22 @@ in @var{modules}."
                                  files)
                        #t))))
 
+(define (xorg-configuration-server-package-path config input path)
+  "Lookup the direct @var{input} in the xorg server package of @var{config}
+and append @var{path} to it."
+  (let* ((server (xorg-configuration-server config))
+         (package (lookup-package-direct-input server input)))
+    (when package (file-append package path))))
+
+(define (xorg-configuration-dri-driver-path config)
+  (xorg-configuration-server-package-path config "mesa" "/lib/dri"))
+
+(define (xorg-configuration-xkb-bin-dir config)
+  (xorg-configuration-server-package-path config "xkbcomp" "/bin"))
+
+(define (xorg-configuration-xkb-dir config)
+  (xorg-configuration-server-package-path config "xkeyboard-config" "/share/X11/xkb"))
+
 (define* (xorg-wrapper #:optional (config (xorg-configuration)))
   "Return a derivation that builds a script to start the X server with the
 given @var{config}.  The resulting script should be used in place of
@@ -365,12 +381,13 @@ given @var{config}.  The resulting script should be used in place of
   (define exp
     ;; Write a small wrapper around the X server.
     #~(begin
-        (setenv "XORG_DRI_DRIVER_PATH" (string-append #$mesa "/lib/dri"))
+        (setenv "XORG_DRI_DRIVER_PATH"
-        (setenv "XKB_BINDIR" (string-append #$xkbcomp "/bin"))
+                #$(xorg-configuration-dri-driver-path config))
+        (setenv "XKB_BINDIR" #$(xorg-configuration-xkb-bin-dir config))
 
         (let ((X (string-append #$(xorg-configuration-server config) "/bin/X")))
           (apply execl X X
-                 "-xkbdir" (string-append #$xkeyboard-config "/share/X11/xkb")
+                 "-xkbdir" #$(xorg-configuration-xkb-dir config)
                  "-config" #$(xorg-configuration->file config)
                  "-configdir" #$(xorg-configuration-directory
                                  (xorg-configuration-modules config))