Add support for out-of-tree platform plugins (#20825)

Summary:
This pull request adds the ability for a platform developer to provide a `"haste"` key under the `"rnpm"` key in their `package.json` which allows the packager to pick up that platform's javascript files. The intent is to remove the need to have custom platforms hardcoded in. This is inspired by the `"jest": { "haste": {} }` key used by jest.

For example, React Native Dom would have an entry like:

```json
{
  "rnpm": {
    "haste": {
      "providesModuleNodeModules": [
        "react-native-dom"
      ],
      "platforms": [
        "dom"
      ]
    }
  }
}
```

Support for more keys (path blacklists perhaps?) could be added in the future.

This succeeds #20662, as per a discussion I had with matthargett.

I've got an open discussion over here as well: https://github.com/react-native-community/discussions-and-proposals/issues/21
Pull Request resolved: https://github.com/facebook/react-native/pull/20825

Differential Revision: D9596429

Pulled By: hramos

fbshipit-source-id: a02f0da0bea8870bdc45d55e23da8ccbc36249f2