Add Store::buildPathsWithResults()

This function is like buildPaths(), except that it returns a vector of
BuildResults containing the exact statuses and output paths of each
derivation / substitution. This is convenient for functions like
Installable::build(), because they then don't need to do another
series of calls to get the outputs of CA derivations. It's also a
precondition to impure derivations, where we *can't* query the output
of those derivations since they're not stored in the Nix database.

Note that PathSubstitutionGoal can now also return a BuildStatus.

src/libstore/build/substitution-goal.hh

@@ -53,6 +53,8 @@ struct PathSubstitutionGoal : public Goal
     /* Content address for recomputing store path */
     std::optional<ContentAddress> ca;
 
+    void done(ExitCode result, BuildResult::Status status);
+
 public:
     PathSubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
     ~PathSubstitutionGoal();