mesa_interface: Move out of GL/internal/

Move it into src/gallium/include/ to make it absolutely clear this is a Mesa detail.
While we're at it, clean up its include sites, including some places
where we can just include kopper_interface.h instead since it includes
mesa_interface.h as its first act.

Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28378>

include/kopper_interface.h

@@ -34,7 +34,7 @@
 #ifndef KOPPER_INTERFACE_H
 #define KOPPER_INTERFACE_H
 
-#include <GL/internal/mesa_interface.h>
+#include "mesa_interface.h"
 #include <vulkan/vulkan_core.h>
 
 typedef struct __DRIkopperExtensionRec          __DRIkopperExtension;