maintain set of files pendingAdd

Kqueue needs to remember which files failed to be added due to being open,
and retry them. This commit gets the data in place for such a retry thread.

Broke KeySource out into its own file, and added Eq and Ord instances
so it can be stored in a Set.

Backend.hs

@@ -6,7 +6,6 @@
  -}
 
 module Backend (
-	B.KeySource(..),
 	list,
 	orderedList,
 	genKey,
@@ -23,6 +22,7 @@ import Config
 import qualified Annex
 import Annex.CheckAttr
 import Types.Key
+import Types.KeySource
 import qualified Types.Backend as B
 
 -- When adding a new backend, import it here and add it to the list.
@@ -54,12 +54,12 @@ orderedList = do
 {- Generates a key for a file, trying each backend in turn until one
  - accepts it.
  -}
-genKey :: B.KeySource -> Maybe Backend -> Annex (Maybe (Key, Backend))
+genKey :: KeySource -> Maybe Backend -> Annex (Maybe (Key, Backend))
 genKey source trybackend = do
 	bs <- orderedList
 	let bs' = maybe bs (: bs) trybackend
 	genKey' bs' source
-genKey' :: [Backend] -> B.KeySource -> Annex (Maybe (Key, Backend))
+genKey' :: [Backend] -> KeySource -> Annex (Maybe (Key, Backend))
 genKey' [] _ = return Nothing
 genKey' (b:bs) source = do
 	r <- B.getKey b source