Fix solver madness when a package version appears only in the optional dependency field.

OPAM is building two different views of the world: in one hand, the universe of package whith hard dependencies; in the other one, the universe of package where all optional dependencies are hard dependencies. The solver uses the first view to give a sensible solution, and then OPAM uses the second view of the world to compute the exact enveloppe of packages to recompile. Having a version string appearing only in the optional dependency makes these two views to not be in sync. I've solved this by adding a dummy package with all the optional dependencies as hard dependencies.

Fix #669