Move ReactFiberTreeReflection to react-reconciler/reflection (#11683)

* Move ReactFiberTreeReflection to react-reconciler/reflection #11659

* Use * for react-reconciler

We don't know the latest local version, and release script currently doesn't bump deps automatically.

* Remove unused field

* Use CommonJS in entry point for consistency

* Undo the CommonJS change

I didn't realize it would break the build.

* Record sizes

* Remove reconciler fixtures

They're unnecessary now that we run real tests on reconciler bundles.