prefer package.json.walnut to package.json.main

lib/package-server.js

@@ -142,6 +142,9 @@ function getApi(pkgConf, pkgDeps, route) {
     try {
       route._apipkg = require(path.join(pkgpath, 'package.json'));
       route._apiname = route._apipkg.name;
+      if (route._apipkg.walnut) {
+        pkgpath += '/' + route.apipkg.walnut;
+      }
       promise = require(pkgpath).create(pkgConf, pkgDeps, myApp);
     } catch(e) {
       reject(e);