Wrong main entry in the generated package.json

The following value is generated:

```
"main": "generators/index.js",
```  

But it should be `generators/app/index.js` as there is no `generators/index.js`. Probably the `projectRoot` passed to `generator-node/generators/app` is wrong:

https://github.com/yeoman/generator-generator/blob/a4abe0c371ee36476ede29dcd29f8f6c8e3b22e6/app/index.js#L69