Solver: soft prefs for optional stanzas are very weak

This should have been part of the main patch for this feature.

This part of the feature takes stanza choice nodes that have been marked
as being soft preferences and makes them "weak", in the same sense (and
via the same mechanism) that we have weak flag choices. With this
change, weak flags choices remain as the weakest, with stanza prefs are
second weakest.

The intention is that we don't try too hard to enable optional stanzas,
ie we don't cause too much additional backtracking.

Author: dcoutts

cabal-install/Distribution/Client/Dependency/Modular/Preference.hs

@@ -324,9 +324,12 @@ deferWeakFlagChoices = trav go
     go (GoalChoiceF xs) = GoalChoiceF (P.sortBy defer xs)
     go x                = x
 
+    -- weak flags go very last, weak stanzas second last
     defer :: Tree a -> Tree a -> Ordering
     defer (FChoice _ _ True _ _) _ = GT
     defer _ (FChoice _ _ True _ _) = LT
+    defer (SChoice _ _ True _) _   = GT
+    defer _ (SChoice _ _ True _)   = LT
     defer _ _                      = EQ
 
 -- Transformation that sorts choice nodes so that