This commit is a follow-up to https://github.com/facebook/react/pull/15604, which explains more of the rationale behind moving React Native to path-based imports and the work needed in the React repository. In that linked PR, the generated renderers were updated but not the shims; this commit updates the shims.
The problem is that FB needs a different copy of the built renderers than the OSS versions so we need a way for FB code to import different modules than in OSS. This was previously done with Haste, but with the removal of Haste from RN, we need another mechanism. Talking with cpojer, we are using a `.fb.js` extension that Metro can be configured to prioritize over `.js`.
This commit generates FB's renderers with the `.fb.js` extension and OSS renderers with just `.js`. This way, FB can internally configure Metro to use the `.fb.js` implementations and OSS will use the `.js` ones, letting us swap out which implementation gets bundled.
Test Plan: Generated the renderers and shims with `yarn build` and then verified that the generated shims don't contain any Haste-style imports. Copied the renderers and shims into RN manually and launched the RNTester app to verify it loads end-to-end. Added `.fb.js` to the extensions in `metro.config.js` and verified that the FB-specific bundles loaded.
* Added new "native-fb" and "native-fabric-fb" bundles.
* Split RN_DEV and RN_PROD bundle types into RN_OSS_DEV, RN_OSS_PROD, RN_FB_DEV, and RN_FB_PROD. (This is a bit redundant but it seemed the least intrusive way of supporting a forked feature flags file for these bundles.)
* Renamed FB_DEV and FB_PROD bundle types to be more explicitly for www (FB_WWW_DEV and FB_WWW_PROD)
* Removed Haste @providesModule headers from the RB-specific RN renderer bundles to avoid a duplicate name conflicts.
* Remove dynamic values from OSS RN feature flags. (Leave them in FB RN feature flags.)
* Updated the sync script(s) to account for new renderer type.
* Move ReactFeatureFlags.js shim to FB bundle only (since OSS bundle no longer needs dynamic values).
* Move build/packages/* to build/node_modules/*
This fixes Node resolution in that folder and lets us require() packages in it in Node shell for manual testing.
* Link fixtures to packages/node_modules
This updates the location and also uses link: instead of file: to avoid Yarn caching the folder contents.
