allow building with aeson >= 2.0

In aeson 2.0, Text has been replaced by the Key type and HashMap by the
KeyMap interface. Accomodating this required adding some CPP in order to
still be able to compile with aeson < 2.0. The required changes were:

* Prevent Key from being re-exported by Utilities.Aeson, as it clashes
  with git-annex's own Key type.

* Fix up convertion from String/Text to Key (or Text in aeson 1.*) in a
  couple of places

* Import Data.Aeson.KeyMap instead of Data.HashMap.Strict, as they are
  mostly API-compatible. insertWith needs to be replaced by unionWith,
  however, as KeyMap lacks the former function.

Messages/JSON.hs

@@ -5,7 +5,7 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
-{-# LANGUAGE OverloadedStrings, GADTs #-}
+{-# LANGUAGE OverloadedStrings, GADTs, CPP #-}
 
 module Messages.JSON (
 	JSONBuilder,
@@ -33,7 +33,12 @@ import Control.Applicative
 import qualified Data.Map as M
 import qualified Data.Vector as V
 import qualified Data.ByteString.Lazy as L
+#if MIN_VERSION_aeson(2,0,0)
+import qualified Data.Aeson.Key as AK
+import qualified Data.Aeson.KeyMap as HM
+#else
 import qualified Data.HashMap.Strict as HM
+#endif
 import System.IO
 import System.IO.Unsafe (unsafePerformIO)
 import Control.Concurrent
@@ -94,7 +99,7 @@ finalize o = addErrorMessage [] o
 
 addErrorMessage :: [String] -> Object -> Object
 addErrorMessage msg o =
-	HM.insertWith combinearray "error-messages" v o
+	HM.unionWith combinearray (HM.singleton "error-messages" v) o
   where
 	combinearray (Array new) (Array old) = Array (old <> new)
 	combinearray new _old = new
@@ -102,7 +107,7 @@ addErrorMessage msg o =
 
 note :: String -> JSONBuilder
 note _ Nothing = Nothing
-note s (Just (o, e)) = Just (HM.insertWith combinelines "note" (toJSON' s) o, e)
+note s (Just (o, e)) = Just (HM.unionWith combinelines (HM.singleton "note" (toJSON' s)) o, e)
   where
 	combinelines (String new) (String old) =
 		String (old <> "\n" <> new)
@@ -127,7 +132,13 @@ add v (Just (o, e)) = case j of
 	j = case v of
 		AesonObject ao -> Object ao
 		JSONChunk l -> object $ map mkPair l
-	mkPair (s, d) = (packString s, toJSON' d)
+	mkPair (s, d) =
+		(
+#if MIN_VERSION_aeson(2,0,0)
+		  AK.fromText $
+#endif
+		  packString s
+		, toJSON' d)
 add _ Nothing = Nothing
 
 complete :: JSONChunk v -> JSONBuilder
@@ -173,7 +184,13 @@ data ObjectMap a = ObjectMap { fromObjectMap :: M.Map String a }
 instance ToJSON' a => ToJSON' (ObjectMap a) where
 	toJSON' (ObjectMap m) = object $ map go $ M.toList m
 	  where
-		go (k, v) = (packString k, toJSON' v)
+		go (k, v) =
+			(
+#if MIN_VERSION_aeson(2,0,0)
+			  AK.fromText $
+#endif
+			  packString k
+			, toJSON' v)
 
 -- An item that a git-annex command acts on, and displays a JSON object about.
 data JSONActionItem a = JSONActionItem