remove unused backend machinery

The only remaining vestiage of backends is different types of keys. These
are still called "backends", mostly to avoid needing to change user interface
and configuration. But everything to do with storing keys in different
backends was gone; instead different types of remotes are used.

In the refactoring, lots of code was moved out of odd corners like
Backend.File, to closer to where it's used, like Command.Drop and
Command.Fsck. Quite a lot of dead code was removed. Several data structures
became simpler, which may result in better runtime efficiency. There should
be no user-visible changes.

Remote.hs

@@ -14,10 +14,10 @@ module Remote (
 	removeKey,
 	hasKey,
 	hasKeyCheap,
+
 	keyPossibilities,
 	keyPossibilitiesTrusted,
 	forceTrust,
-
 	remoteTypes,
 	genList,
 	byName,
@@ -25,6 +25,8 @@ module Remote (
 	remotesWithUUID,
 	remotesWithoutUUID,
 	prettyPrintUUIDs,
+	showTriedRemotes,
+	showLocations,
 
 	remoteLog,
 	readRemoteLog,
@@ -40,6 +42,7 @@ import Data.List
 import qualified Data.Map as M
 import Data.Maybe
 import Data.Char
+import Data.String.Utils
 
 import qualified Branch
 import Types
@@ -49,6 +52,7 @@ import qualified Annex
 import Config
 import Trust
 import LocationLog
+import Messages
 
 import qualified Remote.Git
 import qualified Remote.S3
@@ -181,9 +185,34 @@ keyPossibilities' withtrusted key = do
 
 	return (sort validremotes, validtrusteduuids)
 
+{- Displays known locations of a key. -}
+showLocations :: Key -> [UUID] -> Annex ()
+showLocations key exclude = do
+	g <- Annex.gitRepo
+	u <- getUUID g
+	uuids <- keyLocations key
+	untrusteduuids <- trustGet UnTrusted
+	let uuidswanted = filteruuids uuids (u:exclude++untrusteduuids) 
+	let uuidsskipped = filteruuids uuids (u:exclude++uuidswanted)
+	ppuuidswanted <- Remote.prettyPrintUUIDs uuidswanted
+	ppuuidsskipped <- Remote.prettyPrintUUIDs uuidsskipped
+	showLongNote $ message ppuuidswanted ppuuidsskipped
+	where
+		filteruuids l x = filter (`notElem` x) l
+		message [] [] = "No other repository is known to contain the file."
+		message rs [] = "Try making some of these repositories available:\n" ++ rs
+		message [] us = "Also these untrusted repositories may contain the file:\n" ++ us
+		message rs us = message rs [] ++ message [] us
+
+showTriedRemotes :: [Remote Annex] -> Annex ()
+showTriedRemotes [] = return ()	
+showTriedRemotes remotes =
+	showLongNote $ "Unable to access these remotes: " ++
+		(join ", " $ map name remotes)
+
 forceTrust :: TrustLevel -> String -> Annex ()
 forceTrust level remotename = do
-	r <- Remote.nameToUUID remotename
+	r <- nameToUUID remotename
 	Annex.changeState $ \s ->
 		s { Annex.forcetrust = (r, level):Annex.forcetrust s }