Dropping libGL/EGL vs DRI driver cross compatibility

Now that Amber exists and glvnd seems to be the route we plan on for
"supporting" old DRI drivers, I'm working toward garbage collecting a lot
of extra layering that we have to support out of tree DRI drivers.  I feel
that the loader interface gets in the way of implementing new EGL
functionality, and obscures how the window system interacts with the
gallium drivers.  If we ensure that we only load a Mesa driver from our
tree, then the window system can just interact with the dri_screen and
pipe_screen directly.

MR is up at https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20069

Anyone see any blocking issues for this?