Update build script to minimally support subpath patterns (#47673)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47673

Update shared monorepo build script:
- Refactor entry point discovery and file iteration
- Add support for "exports" subpath patterns (skip considering as entry point with warning)

This is required for the incoming migration of `react-native-codegen` to this build setup (D51465053).

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D66121262

fbshipit-source-id: 0deb2c2b26de442ee348ef9de54a1fa144368b6f