* 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.hh

@@ -37,10 +37,11 @@ public:
 
     Path queryDeriver(const Path & path);
     
-    PathSet querySubstitutablePaths();
-    
     bool hasSubstitutes(const Path & path);
     
+    bool querySubstitutablePathInfo(const Path & path,
+        SubstitutablePathInfo & info);
+    
     Path addToStore(const Path & srcPath, bool fixed = false,
         bool recursive = false, string hashAlgo = "",
         PathFilter & filter = defaultPathFilter);