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

.eslintrc.js

@@ -60,7 +60,10 @@ module.exports = {
       },
     },
     {
-      files: ['packages/react-native/Libraries/**/*.js'],
+      files: [
+        'packages/react-native/Libraries/**/*.js',
+        'packages/react-native/src/**/*.js',
+      ],
       rules: {
         '@react-native/platform-colors': 2,
         '@react-native/specs/react-native-modules': 2,

jest.config.js

@@ -56,7 +56,10 @@ module.exports = {
     'denodeify',
   ],
   testEnvironment: 'node',
-  collectCoverageFrom: ['packages/react-native/Libraries/**/*.js'],
+  collectCoverageFrom: [
+    'packages/react-native/Libraries/**/*.js',
+    'packages/react-native/src/**/*.js',
+  ],
   coveragePathIgnorePatterns: [
     '/__tests__/',
     '/vendor/',

packages/react-native/package.json

@@ -79,6 +79,7 @@
     "sdks/hermes-engine",
     "sdks/hermesc",
     "settings.gradle.kts",
+    "src",
     "template.config.js",
     "template",
     "!template/node_modules",