add api endpoint: /build/<id>/constituents

Returns a list of constituent builds

t/jobs/aggregate.nix

@@ -0,0 +1,26 @@
+with import ./config.nix;
+{
+  a =
+    mkDerivation {
+      name = "empty-dir-a";
+      builder = ./empty-dir-builder.sh;
+    };
+
+  b =
+    mkDerivation {
+      name = "empty-dir-b";
+      builder = ./empty-dir-builder.sh;
+    };
+
+  aggregate =
+    mkDerivation {
+      name = "aggregate";
+      builder = ./empty-dir-builder.sh; # doesn't matter, just needs to pass a build
+
+      _hydraAggregate = true;
+      constituents = [
+        "a"
+        "b"
+      ];
+    };
+}