* Make nix-env --dry-run print the paths to be substituted correctly

again.  (After the previous substituter mechanism refactoring I
  didn't update the code that obtains the references of substitutable
  paths.)  This required some refactoring: the substituter programs
  are now kept running and receive/respond to info requests via
  stdin/stdout.

src/libstore/remote-store.cc

@@ -213,6 +213,13 @@ bool RemoteStore::hasSubstitutes(const Path & path)
 }
 
 
+bool RemoteStore::querySubstitutablePathInfo(const Path & path,
+    SubstitutablePathInfo & info)
+{
+    throw Error("not implemented");
+}
+
+
 Hash RemoteStore::queryPathHash(const Path & path)
 {
     writeInt(wopQueryPathHash, to);
@@ -256,12 +263,6 @@ Path RemoteStore::queryDeriver(const Path & path)
 }
 
 
-PathSet RemoteStore::querySubstitutablePaths()
-{
-    throw Error("not implemented");
-}
-
-
 Path RemoteStore::addToStore(const Path & _srcPath, bool fixed,
     bool recursive, string hashAlgo, PathFilter & filter)
 {