Rewrite buildForest in terms of graphs (and strongly-connected

components)

This commit rewrites the logic behind `buildForest` to build first a
graph, discovering loops in there (via the strongly-connected components
machinery) and finally converting the DAG (freshly-made so) into a
Forest.

src/Gargantext/Core/NodeStory/Types.hs

@@ -198,7 +198,7 @@ data BuildForestError
   = -- We found a loop, something that shouldn't normally happen if the calling
     -- code is correct by construction, but if that does happen, the value will
     -- contain the full path to the cycle.
-    BFE_loop_detected !(Set VisitedNode)
+      BFE_loop_detected !(Set VisitedNode)
   deriving (Show, Eq)
 
 instance ToHumanFriendlyError BuildForestError where

test/Test/Ngrams/Query.hs

@@ -235,7 +235,7 @@ testFlat05 = do
 testForestSearchProp :: Property
 testForestSearchProp = forAll arbitrary $ \(AcyclicTableMap ngramsTable el) -> do
   case searchTableNgrams (Versioned 0 ngramsTable) (searchQuery el) of
-    Left (BFE_loop_detected err) -> fail (T.unpack $ renderLoop err)
+    Left (BFE_loop_detected err)    -> fail (T.unpack $ renderLoop err)
     Right res -> res ^. vc_data `shouldSatisfy` (any (containsTerm (_ne_ngrams el)) . getNgramsTable)
   where
    searchQuery term = NgramsSearchQuery {

test/Test/Offline/Ngrams.hs

@@ -315,8 +315,8 @@ testLoopBreaker_02 =
   let t1 = Map.fromList [ ( "foo", mkMapTerm "foo" & ne_children .~ mSetFromList ["bar"])
                         , ( "bar", mkMapTerm "bar" & ne_children .~ mSetFromList ["foo"])
                         ]
-  in (buildForestOrBreakLoop t1) `compareForestVisually` [r|
-         foo
+  in (pruneForest $ buildForestOrBreakLoop t1) `compareForestVisually` [r|
+         bar
          |
-         `- bar
+         `- foo
 |]