switch from hslogger to purpose-built Utility.Debug

This uses a DebugSelector, rather than debug levels, which will allow
for a later option like --debug-from=Process to only
see debuging about running processes.

The module name that contains the thing being debugged is used as the
DebugSelector (in most cases; does not need to be a hard and fast rule).
Debug calls were changed to add that. hslogger did not display
that first parameter to debugM, but the DebugSelector does get
displayed.

Also fastDebug will allow doing debugging in places that are used in
tight loops, with the DebugSelector coming from the Annex Reader
essentially for free. Not done yet.

Backend/External.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE OverloadedStrings #-}
 
 module Backend.External (makeBackend) where
 
@@ -16,6 +17,7 @@ import Types.Key
 import Types.Backend
 import Types.KeySource
 import Utility.Metered
+import Utility.Debug
 import qualified Utility.SimpleProtocol as Proto
 
 import qualified Data.ByteString as S
@@ -23,7 +25,6 @@ import qualified Data.Map.Strict as M
 import Data.Char
 import Control.Concurrent
 import System.IO.Unsafe (unsafePerformIO)
-import System.Log.Logger (debugM)
 
 newtype ExternalBackendName = ExternalBackendName S.ByteString
 	deriving (Show, Eq, Ord)
@@ -141,7 +142,7 @@ handleRequest st req whenunavail responsehandler =
 		warning ("external special remote error: " ++ err)
 		whenunavail
 	handleExceptionalMessage loop (DEBUG msg) = do
-		liftIO $ debugM "external" msg
+		liftIO $ debug "Backend.External" msg
 		loop
 
 withExternalAddon :: ExternalState -> a -> (ExternalAddonProcess -> a) -> a