Solve more slot-operator conflicts (531656)

Add some heuristics to handle slot conflicts triggered by interaction
of slot-operator dependencies with dependencies like those of labgl:

		ocaml:= || ( labltk <ocaml-4.02 )

The new heuristics involve some behavior modifications in the depgraph
_solve_non_slot_operator_slot_conflicts method and in dep_zapdeps. The
dep_zapdeps changes affect the behavior of _select_atoms_probe calls
in the depgraph _slot_operator_update_probe method.

X-Gentoo-Bug: 531656
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=531656