Fix path based dependencies to be ignored for version selections.

Fix unit tests of DependencyResolver.

Fix resolution of optional and path based dependencies.

Correct copyright notices.

Fix handling of overriding selections and let printUpgradesOnly fail gracefully.

Fix dependency resolution for outdated invalid dependencies. ...

Now the invalid dependency is simply ignored as long as it isn't actually chosen.

Move DependencyResolver to its own module and reorder Version<->Dependency.