Sync React with Haste-style imports rewritten to use path-based imports instead (#25100)

Summary:
**This is a manual React sync to replace Haste names with paths so that the removal of Haste is not blocked on a normal React sync. This is a one-time special case; in the future, React will generate renderers that use paths instead of Haste.**

This commit uses the same base commit of React that's currently in RN (React ec6691a68716bc59291746fc62f374a56fb435c9) plus a commit in the React repo that removes Haste-style imports from the renderer (61f62246c8cfb76a4a19d1661eeaa5822ec37b36) and a commit to make the shims import from `implementations` (facebook/react#15786).

I built React in the React repo with `yarn build` and copied over the `oss` directory into one called `implementations` and removed the `*.fb.js` files.

## Changelog

[General] [Changed] - Sync React with Haste-style imports rewritten to use path-based imports instead
Pull Request resolved: facebook/react-native#25100

Reviewed By: yungsters

Differential Revision: D15575646

Pulled By: cpojer

fbshipit-source-id: adf25f9826b71729043b65ba1afd20d14d8c19c4

Author: ide

Libraries/Renderer/REVISION

@@ -1 +1 @@
-ec6691a68716bc59291746fc62f374a56fb435c9
+87d375afd0d9394646a4bcddcebb7b15ffaa7e9e