Use the hash modulo in the derivation outputs

Rather than storing the derivation outputs as `drvPath!outputName` internally,
store them as `drvHashModulo!outputName` (or `outputHash!outputName` for
fixed-output derivations).

This makes the storage slightly more opaque, but enables an earlier
cutoff in cases where a fixed-output dependency changes (but keeps the
same output hash) − same as what we already do for input-addressed
derivations.

src/libexpr/primops.cc

@@ -1107,7 +1107,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                 // Shouldn't happen as the toplevel derivation is not CA.
                 assert(false);
             },
-            [&](UnknownHashes) {
+            [&](DeferredHash _) {
                 for (auto & i : outputs) {
                     drv.outputs.insert_or_assign(i,
                         DerivationOutput {