I tried, and when I try to manually install some package already present, I get a message like this:
apt-get install xserver-xorg-core locales xserver-xorg-video-modesetting
Reading package lists... Done
Building dependency tree
Reading state information... Done
xserver-xorg-video-modesetting is already the newest version.
xserver-xorg-video-modesetting set to manually installed.
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
locales : Depends: libc-bin (> 2.24) but 2.23-5+b1 is to be installed
xserver-xorg-core : Conflicts: xserver-xorg-video-modesetting
xserver-xorg-video-modesetting : Depends: xorg-video-abi-18
E: Unable to correct problems, you have held broken packages.
I believe this has something to do about the fact, that you mentioned, of unstable packages... I guess I'll have to try each one...