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/Variety.hs

@@ -0,0 +1,35 @@
+{- git-annex backend varieties
+ -
+ - Copyright 2012-2024 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Backend.Variety where
+
+import qualified Data.Map as M
+
+import Annex.Common
+import Types.Key
+import Types.Backend
+import qualified Backend.External
+
+-- When adding a new backend, import it here and add it to the builtinList.
+import qualified Backend.Hash
+import qualified Backend.WORM
+import qualified Backend.URL
+
+{- Regular backends. Does not include externals or VURL. -}
+regularBackendList :: [Backend]
+regularBackendList = Backend.Hash.backends 
+	++ Backend.WORM.backends 
+	++ Backend.URL.backends
+
+makeVarietyMap :: [Backend] -> M.Map KeyVariety Backend
+makeVarietyMap l = M.fromList $ zip (map backendVariety l) l
+
+maybeLookupBackendVarietyMap :: KeyVariety -> M.Map KeyVariety Backend -> Annex (Maybe Backend)
+maybeLookupBackendVarietyMap (ExternalKey s hasext) _varitymap =
+	Just <$> Backend.External.makeBackend s hasext
+maybeLookupBackendVarietyMap v varietymap = 
+	pure $ M.lookup v varietymap