change header structure rn deps in XCFrameworks (#50085)

Summary:
Headers are currently copied into each arch in the final xcframework. This is not necessary and will cause a lot of duplication since these files are the same for all archs.

This commit fixes this by only copying headers when we build the final XCFramework:

- ReactNativeDependencies.podspec: Changed the prepare script to be more resilient to different header structs, since we have multiple ways of packaging our tarballs locally and on the servers
- build.js: Removed copying headers when building frameworks
- compose-framework.js: Added copying headers once to the root of the XCFramework.
- rndependencies.rb: updated docs with correct ENV vars

bypass-github-export-checks

## Changelog:

[INTERNAL] - Changes the header structure in our XCFramework to avoid duplication

Pull Request resolved: https://github.com/facebook/react-native/pull/50085

Test Plan: Run RNTester with RCT_USE_RN_DEPS=1 to use prebuilt RN Deps.

Reviewed By: javache

Differential Revision: D71385183

Pulled By: cipolleschi

fbshipit-source-id: 160d56dea7f61d1b8d0a45b80f6c0789647358e6