building up to React Core (#50766)

Summary:
- Added multiple targets to Package.swift
- WIP: Builds up untill we need to touch files in React/Base

## Changelog:

[INTERNAL] - WIP: prebuilding using Swift packages

Pull Request resolved: https://github.com/facebook/react-native/pull/50766

Test Plan: This will be tested in a diff in the stack.

Reviewed By: cortinico

Differential Revision: D74386522

Pulled By: cipolleschi

fbshipit-source-id: 7bac3c21a362c4ef79d0104727cdd3494419012f