implement isCryptographicallySecureKey for VURL

Considerable difficulty to work around an import cycle. Had to move the
list of backends (except for VURL) to Backend.Variety to VURL could use
it.

Sponsored-by: Kevin Mueller on Patreon

Backend/URL.hs

@@ -1,5 +1,4 @@
-{- git-annex "URL" and "VURL" backends -- keys whose content is
- - available from urls.
+{- git-annex URL backend -- keys whose content is available from urls.
  -
  - Copyright 2011-2024 Joey Hess <id@joeyh.name>
  -
@@ -15,10 +14,9 @@ import Annex.Common
 import Types.Key
 import Types.Backend
 import Backend.Utilities
-import Logs.EquivilantKeys
 
 backends :: [Backend]
-backends = [backendURL, backendVURL]
+backends = [backendURL]
 
 backendURL :: Backend
 backendURL = Backend
@@ -31,25 +29,8 @@ backendURL = Backend
 	-- The content of an url can change at any time, so URL keys are
 	-- not stable.
 	, isStableKey = const False
-	, isCryptographicallySecure = pure False
-	}
-
-backendVURL :: Backend
-backendVURL = Backend
-	{ backendVariety = VURLKey
-	, genKey = Nothing
-	, verifyKeyContent = Nothing -- TODO
-	, verifyKeyContentIncrementally = Nothing -- TODO
-	, canUpgradeKey = Nothing
-	, fastMigrate = Nothing
-	-- Even if a hash is recorded on initial download from the web and
-	-- is used to verify every subsequent transfer including other
-	-- downloads from the web, in a split-brain situation there
-	-- can be more than one hash and different versions of the content.
-	-- So the content is not stable.
-	, isStableKey = const False
-	, isCryptographicallySecure = pure False 
-	-- TODO it is when all recorded hashes are
+	, isCryptographicallySecure = False
+	, isCryptographicallySecureKey = const (pure False)
 	}
 
 {- Every unique url has a corresponding key. -}