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-result.hh

@@ -28,6 +28,7 @@ struct BuildResult
         LogLimitExceeded,
         NotDeterministic,
         ResolvesToAlreadyValid,
+        NoSubstituters,
     } status = MiscFailure;
     std::string errorMsg;
 
@@ -63,15 +64,26 @@ struct BuildResult
        non-determinism.) */
     bool isNonDeterministic = false;
 
+    /* For derivations, the derivation path and the wanted outputs. */
+    std::optional<StorePath> drvPath;
     DrvOutputs builtOutputs;
 
+    /* For substitutions, the substituted path. */
+    std::optional<StorePath> outPath;
+
     /* The start/stop times of the build (or one of the rounds, if it
        was repeated). */
     time_t startTime = 0, stopTime = 0;
 
-    bool success() {
+    bool success()
+    {
         return status == Built || status == Substituted || status == AlreadyValid || status == ResolvesToAlreadyValid;
     }
+
+    void rethrow()
+    {
+        throw Error("%s", errorMsg);
+    }
 };
 
 }