Make the greedy slot atom code from bug #150361 look ahead for conflicts

Make the greedy slot atom code from bug #150361 look ahead for conflicts
between the packages that will be pulled in. If a conflict is discovered,
drop the greedy slot atom for the older slot. This solves a problem reported
by kde4 users when running `emerge -u <atom>` for a package that has the
kdeprefix flag disabled. In this case, the newer slot blocks the older slot.
In order to allow the older slot to be automatically uninstalled, the atom
for the older slot needs to be excluded from the graph. Thanks to Maciej
Mrozowski for testing this patch.

svn path=/main/trunk/; revision=12427