Derivations can output "text-hashed" data

In particular, this means that derivations can output derivations. But
that ramification isn't (yet!) useful as we would want, since there is
no way to have a dependent derivation that is itself a dependent
derivation.

src/libstore/remote-store.hh

@@ -68,6 +68,7 @@ public:
         Source & dump,
         const string & name,
         ContentAddressMethod caMethod,
+        HashType hashType,
         const StorePathSet & references,
         RepairFlag repair);