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In aeson 2.0, Text has been replaced by the Key type and HashMap by the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

Types/MetaData.hs

@@ -7,6 +7,7 @@
 
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE CPP #-}
 
 module Types.MetaData (
 	MetaData(..),
@@ -51,6 +52,9 @@ import Utility.QuickCheck
 import Utility.Aeson
 import Types.UUID
 
+#if MIN_VERSION_aeson(2,0,0)
+import qualified Data.Aeson.Key as AK
+#endif
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as E
 import qualified Data.Set as S
@@ -67,7 +71,13 @@ newtype MetaData = MetaData (M.Map MetaField (S.Set MetaValue))
 instance ToJSON' MetaData where
 	toJSON' (MetaData m) = object $ map go (M.toList m)
 	  where
-		go (MetaField f, s) = (CI.original f, toJSON' s)
+		go (MetaField f, s) =
+			(
+#if MIN_VERSION_aeson(2,0,0)
+			  AK.fromText $
+#endif
+			  CI.original f
+			, toJSON' s)
 
 instance FromJSON MetaData where
 	parseJSON (Object o) = do