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.

src/ReactTestRenderer.js

@@ -12,7 +12,7 @@ import type {FiberRoot} from 'react-reconciler/src/ReactFiberRoot';
 
 import ReactFiberReconciler from 'react-reconciler';
 import {batchedUpdates} from 'events/ReactGenericBatching';
-import {findCurrentFiberUsingSlowPath} from 'shared/ReactFiberTreeReflection';
+import {findCurrentFiberUsingSlowPath} from 'react-reconciler/reflection';
 import emptyObject from 'fbjs/lib/emptyObject';
 import {
   Fragment,