Libglx is optional, in case of nvidia it's replaced by something in their driver.
As drivers depend on libgl through dri already, I don't see the need for libgl dependency for xorg-server.

So let's take a look:
In order to have the server working one can select between nvidia's libglx or the xorg-server one
- package "nvidia-libgl", provides "libgl" contains the file libglx.so
- package "mesa-libgl", provides "libgl" contains the files (symlinks) for libglx.so and libGL.so

So regardless of how I look at it, in order to have a working xorg-server (and a nice prompt at it's install time) one needs a "libgl" provider.
Am I missing something ?

Thanks

P.S.
Regarding the dri/mesa/mesa-libgl dependency I believe it's a bit off yet that's another issue.

I've not checked but i don't think glx is mandatory to have a working xorg-server, perhaps i'm wrong?

You are absolutely correct. For glamor users (radeonsi + nouveau/maxwell) one needs libEGL, and the above two packages provide it.
So regardless of how I look at it, adding 'depends "libgl"' for the package seems like the correct thing.

I will add a dependency on libgl for xorg-server, because glamor need it.

It should be available with xorg-server-1.16.1