Add support for src directory in react-native (#42385)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42385

This adds support for having JS files in a `src` directory within the `react-native` package. The plan is to have 2 subdirectories there:
* `react-native/src/private` for private modules, with any nested directories (e.g.: `react-native/src/private/dom/nodes/ReadOnlyNode.js`).
* `react-native/src/public` for public modules, without nested directories. The plan is that the individual modules created in this directory will be public through the index module or directly via something like `react-native/View` (mapped to `react-native/src/public/View`, or a `dist` directory in the published npm package—details TBD).

The enforcement of private modules being inaccessible from outside the `react-native` package will be added soon by huntie.

Changelog: [internal]

Reviewed By: huntie

Differential Revision: D52875999

fbshipit-source-id: 914ed806f2cb86857e2cb7b760292c2190b5b14e