implement URL to VURL migration

This needs the content to be present in order to hash it. But it's not
possible for a module used by Backend.URL to call inAnnex because that
would entail a dependency loop. So instead, rely on the fact that
Command.Migrate calls inAnnex before performing a migration.

But, Command.ExamineKey calls fastMigrate and the key may or may not
exist, and it's not wanting to actually perform a migration in any case.
To handle that, had to add an additional value to fastMigrate to
indicate whether the content is inAnnex.

Factored generateEquivilantKey out of Remote.Web.

Note that migrateFromURLToVURL hardcodes use of the SHA256E backend.
It would have been difficult not to, given all the dependency loop
issues. But --backend and annex.backend are used to tell git-annex
migrate to use VURL in any case, so there's no config knob that
the user could expect to configure that.

Sponsored-by: Brock Spratlen on Patreon

Backend.hs

@@ -38,11 +38,6 @@ import qualified Backend.VURL
 builtinList :: [Backend]
 builtinList = regularBackendList ++ Backend.VURL.backends
 
-{- The default hashing backend. This must use a cryptographically secure
- - hash. -}
-defaultHashBackend :: Backend
-defaultHashBackend = Prelude.head builtinList
-
 {- Backend to use by default when generating a new key. Takes git config
  - and --backend option into account. -}
 defaultBackend :: Annex Backend