Move the ConfiguredPackage validation out of the InstallPlan module

The InstallPlan can be generalised by abstracting over the specific
package types. The only thing that really relies on a lot of the
details of the concrete ConfiguredPackage type is the bit that
validates them individually (as opposed to validating packages
within the plan in relation to other packages, graph structure etc).

So as a prelude to generalising the InstallPlan, move the checks
on the ConfiguredPackage into the Dependency module and use them
when checking the output of the solver.