releaseTools: Add ‘aggregate’ function

An aggregate is a trivial build that depends on other builds.  This is
intended to provide a declarative replacement of Hydra's "view"
mechanism.

For instance, you can define an aggregate named "critical" that
depends on a selected set of jobs:

  critical = releaseTools.aggregate
    { name = "foo-${tarball.version}";
      members =
        [ tarball
          build.x86_64-linux
          ...
        ];
      meta.description = "Release-critical builds";
    };

The "critical" build will only succeed if all its members
(dependencies) succeed.

pkgs/build-support/release/default.nix

@@ -39,4 +39,14 @@ rec {
     { inherit stdenv vmTools checkinstall;
     } // args);
 
+  aggregate =
+    { name, members, meta ? { } }:
+    pkgs.runCommand name
+      { inherit members meta;
+        _hydraAggregate = true;
+      }
+      ''
+        echo $members > $out
+      '';
+
 }