Solver: Represent an unknown package with a failure node.

Previously, the solver represented an unknown package with an empty PChoice
node, because an unknown package is like a package with no versions.  Now that
--reject-unconstrained-dependencies also needs to reject whole packages, it is
simpler to handle both types of failures similarly, and use a failure node for
both.  Another advantage of using a failure node now is that the node can store
more information about why the package was rejected.  The explicit node also
simplifies generating the solver log.

This commit treats a failure node under a goal choice as a backjump in
D.S.Modular.Explore.exploreLog, which fixes #5502.