Fix a bug that caused important debugging info to be deleted in modul…

…ar solver.

The question is whether or not cabal-install will tell you why it tries
a particular goal (i.e., because it's the dependency of something else).

This regression was introduced between 0.14.0 and 1.16.0 of
cabal-install in order to fix a space leak. Restoring the information
actually costs some space again (although not as much as was required
back in 0.14.0).  I think the extra info is worth it though.

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

@@ -211,7 +211,8 @@ avoidReinstalls p = trav go
 firstGoal :: Tree a -> Tree a
 firstGoal = trav go
   where
-    go (GoalChoiceF xs) = casePSQ xs (GoalChoiceF xs) (\ _ t _ -> out t)
+    go (GoalChoiceF xs) = -- casePSQ xs (GoalChoiceF xs) (\ _ t _ -> out t) -- more space efficient, but removes valuable debug info
+                          casePSQ xs (GoalChoiceF (fromList [])) (\ g t _ -> GoalChoiceF (fromList [(g, t)]))
     go x                = x
     -- Note that we keep empty choice nodes, because they mean success.