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In aeson 2.0, Text has been replaced by the Key type and HashMap by the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.

Remote.hs

@@ -5,7 +5,7 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
-{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE OverloadedStrings, CPP #-}
 
 module Remote (
 	Remote,
@@ -63,6 +63,9 @@ module Remote (
 
 import Data.Ord
 import Data.String
+#if MIN_VERSION_aeson(2,0,0)
+import qualified Data.Aeson.Key as AK
+#endif
 import qualified Data.Map as M
 import qualified Data.Vector as V
 
@@ -239,11 +242,17 @@ prettyPrintUUIDsWith optfield header descm showval uuidvals = do
 			Nothing -> s
 			Just val -> val ++ ": " ++ s
 	jsonify hereu (u, optval) = object $ catMaybes
-		[ Just (packString "uuid", toJSON' (fromUUID u :: String))
-		, Just (packString "description", toJSON' $ finddescription u)
-		, Just (packString "here", toJSON' $ hereu == u)
+		[ Just ("uuid", toJSON' (fromUUID u :: String))
+		, Just ("description", toJSON' $ finddescription u)
+		, Just ("here", toJSON' $ hereu == u)
 		, case (optfield, optval) of
-			(Just field, Just val) -> Just (packString field, toJSON' val)
+			(Just field, Just val) -> Just
+				(
+#if MIN_VERSION_aeson(2,0,0)
+				  AK.fromText $
+#endif
+				  packString field
+				, toJSON' val)
 			_ -> Nothing
 		]