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.

Command/Get.hs

@@ -8,7 +8,6 @@
 module Command.Get where
 
 import Command
-import qualified Backend
 import qualified Annex
 import qualified Remote
 import Types
@@ -24,7 +23,7 @@ seek :: [CommandSeek]
 seek = [withFilesInGit start]
 
 start :: CommandStartString
-start file = isAnnexed file $ \(key, backend) -> do
+start file = isAnnexed file $ \(key, _) -> do
 	inannex <- inAnnex key
 	if inannex
 		then stop
@@ -32,14 +31,52 @@ start file = isAnnexed file $ \(key, backend) -> do
 			showStart "get" file
 			from <- Annex.getState Annex.fromremote
 			case from of
-				Nothing -> next $ perform key backend
+				Nothing -> next $ perform key
 				Just name -> do
 					src <- Remote.byName name
 					next $ Command.Move.fromPerform src False key
 
-perform :: Key -> Backend Annex -> CommandPerform
-perform key backend = do
-	ok <- getViaTmp key (Backend.retrieveKeyFile backend key)
+perform :: Key -> CommandPerform
+perform key = do
+	ok <- getViaTmp key (getKeyFile key)
 	if ok
 		then next $ return True -- no cleanup needed
 		else stop
+
+{- Try to find a copy of the file in one of the remotes,
+ - and copy it to here. -}
+getKeyFile :: Key -> FilePath -> Annex Bool
+getKeyFile key file = do
+	remotes <- Remote.keyPossibilities key
+	if null remotes
+		then do
+			showNote "not available"
+			Remote.showLocations key []
+			return False
+		else trycopy remotes remotes
+	where
+		trycopy full [] = do
+			Remote.showTriedRemotes full
+			Remote.showLocations key []
+			return False
+		trycopy full (r:rs) = do
+			probablythere <- probablyPresent r
+			if probablythere
+				then docopy r (trycopy full rs)
+				else trycopy full rs
+		-- This check is to avoid an ugly message if a remote is a
+		-- drive that is not mounted.
+		probablyPresent r =
+			if Remote.hasKeyCheap r
+				then do
+					res <- Remote.hasKey r key
+					case res of
+						Right b -> return b
+						Left _ -> return False
+				else return True
+		docopy r continue = do
+			showNote $ "from " ++ Remote.name r ++ "..."
+			copied <- Remote.retrieveKeyFile r key file
+			if copied
+				then return True
+				else continue