hubot-tweeter package failing while community script succeeds

When using the hubot-tweeter package, I get this:

```
2013-11-06T19:00:55.881546+00:00 heroku[web.1]: State changed from starting to up
2013-11-06T19:00:56.291171+00:00 app[web.1]: [Wed Nov 06 2013 19:00:56 GMT+0000 (UTC)] ERROR Error loading scripts from npm package - Error: Cannot find module 'hubot-tweeter'
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at Function.Module._resolveFilename (module.js:338:15)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at Module.require (module.js:362:17)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at Function.Module._load (module.js:280:25)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at require (module.js:378:17)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at Robot.loadExternalScripts (/app/node_modules/hubot/src/robot.coffee:222:11, <js>:179:27)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at fs.readFile (fs.js:176:14)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at Object.oncomplete (fs.js:297:15)
2013-11-06T19:00:56.291171+00:00 app[web.1]:   at _ref (/app/node_modules/hubot/bin/hubot:113:13, <js>:122:28)
2013-11-06T19:00:56.291171+00:00 app[web.1]: 
2013-11-06T19:00:57.516589+00:00 heroku[web.1]: Process exited with status 1
2013-11-06T19:00:57.530581+00:00 heroku[web.1]: State changed from up to crashed
```

Using all the same settings, but moving the script config from `external-scripts.json` to `hubot-scripts.json` (with requisite format change), worked fine.

Couldn't make time to test, but seems hubot-tweeter doesn't make use of the `main` key in `packages.json` like other hubot packages do, so that might be the reason.