Fix a case in the new solver pruning pass

It is actually possible for pruning to eliminate required packages
because it's possible for it to propagate all the way up to one of
the initial targets.

cabal-install/Distribution/Client/Dependency/TopDown.hs

@@ -354,8 +354,7 @@ pruneBottomUp platform comp constraints =
         Satisfiable cs' [pkgid]| packageId pkg == pkgid
                          -> Step (ExcludeUnconfigurable pkgid) (rest cs')
         Satisfiable _ _  -> impossible
-        Unsatisfiable    -> impossible
-        ConflictsWith _  -> Fail $ ConfigureFailed pkg
+        _                -> Fail $ ConfigureFailed pkg
                               [ (dep, Constraints.conflicting cs dep)
                               | dep <- missing ]