Register the realisations for unresolved drvs

Once a build is done, get back to the original derivation, and register
all the newly built outputs for this derivation.

This allows Nix to work properly with derivations that don't have all
their build inputs available − thus allowing garbage collection and
(once it's implemented) binary substitution

src/libstore/build/derivation-goal.hh

@@ -48,6 +48,9 @@ struct DerivationGoal : public Goal
     /* The path of the derivation. */
     StorePath drvPath;
 
+    /* The path of the corresponding resolved derivation */
+    std::optional<BasicDerivation> resolvedDrv;
+
     /* The specific outputs that we need to build.  Empty means all of
        them. */
     StringSet wantedOutputs;