Fix build after unifying RNCore in FBReactNativeSpec (#51766)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51766

Yesterday we landed [95ed8a6123](https://github.com/facebook/react-native/commit/95ed8a6123024afd03227358ab4b165fdf87b270) where we unified `rncore` and `FBReactNativeSpec` into the latter.

This of course broke the current setup of SwiftPM. This diff fixes it.

## Changelog:
[Internal] - Exclude rncore components

Reviewed By: javache

Differential Revision: D75863335

fbshipit-source-id: ad2739bed5645e41048a8bc466736712db18c3b6