Why doesn't the driver package depend on the kernel module package?
The driver can't depend on the kernel module but only recommend it,
otherwise you can't install the driver until you have the kernel module
package ready and deployed in a place where apt can find it;
the latter is a nontrivial problem by itself.
Also, technically the driver does not actually depend on the kernel module,
as it works fine without (but with no 3D acceleration of course).
Why doesn't the driver package provide the “libgl1” virtual package,
instead of diverting /usr/X11R6/lib/libGL.so.1 away?
Because the fglrx driver doesn't provide full development headers and libraries.
By just diverting away libGL, you are able to install, for example,
the xlibmesa-gl and xlibmesa-gl-dev packages.
Why doesn't the driver package alter the XFree86/X.Org configuration?
I'm reluctant to do so, for fear of breaking the existing XFree86/X.Org configuration.
The driver package triggers a lintian error, ldconfig-symlink-missing-for-shlib.
Can you fix it?
Yes, but I would also have to divert the symlink “libGL.so.1” in addition
to the actual library “libGL.so.1.2” and I'm not sure this makes sense.
I believe that the call to ldconfig in the postinst,
automatically added by dh_shlibdeps, is enough.