Bug#381857:

["Roberto Piscitello" <robepisc@gmail.com>]

The problem is a diversion that an old installation of
nvidia-glx-legacy left on my system.
Now I don't have nvidia-glx-legacy installed anymore, but in my
/var/lib/dpkg/diversions I found these lines:

/usr/lib/xorg/modules/extensions/libGLcore.a
/usr/lib/nvidia/libGLcore.a.xlibmesa
nvidia-glx-legacy
/usr/lib/xorg/modules/extensions/libglx.a
/usr/lib/nvidia/libglx.a.xlibmesa
nvidia-glx-legacy
/usr/lib/xorg/modules/extensions/libglx.so
/usr/lib/nvidia/libglx.so.xlibmesa
nvidia-glx-legacy

Dpkg looks for those /usr/lib/nvidia/* files, can't find them and bails out.
I removed those diversions with:
dpkg-divert --rename --remove /usr/lib/xorg/modules/extensions/libGLcore.a
dpkg-divert --rename --remove /usr/lib/xorg/modules/extensions/libglx.a
dpkg-divert --rename --remove /usr/lib/xorg/modules/extensions/libglx.so

Then xserver-xorg-core installed without problems.