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/Utilities.hs

@@ -1,6 +1,6 @@
 {- git-annex backend utilities
  -
- - Copyright 2012-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}