* Propagate the deriver of a path through the substitute mechanism.

* Removed some dead code (successor stuff) from nix-push.
* Updated terminology in the tests (store expr -> drv path).
* Check that the deriver is set properly in the tests.

src/nix-store/main.cc

@@ -280,6 +280,7 @@ static void opSubstitute(Strings opFlags, Strings opArgs)
         PathSet references;
         getline(cin, srcPath);
         if (cin.eof()) break;
+        getline(cin, sub.deriver);
         getline(cin, sub.program);
         string s;
         int n;