mesa: add driversdev output and test that dev doesn't depend on drivers

OpenGL applications should not depend on the Mesa drivers, since these
are supposed to be loaded at runtime from /run/opengl-driver using the
glvnd library.  Furthermore, the drivers output has a large closure
since it depends on LLVM.

The only dependency from dev to drivers came from the xatracker
pkg-config file.  This commit moves this file into a new
output called driversdev.

pkgs/servers/x11/xorg/overrides.nix

@@ -433,7 +433,7 @@ self: super:
   });
 
   xf86videovmware = super.xf86videovmware.overrideAttrs (attrs: {
-    buildInputs =  attrs.buildInputs ++ [ mesa llvm ]; # for libxatracker
+    buildInputs =  attrs.buildInputs ++ [ mesa mesa.driversdev llvm ]; # for libxatracker
     meta = attrs.meta // {
       platforms = ["i686-linux" "x86_64-linux"];
     };