Move VirtualCollection components into react-native-github (#56256)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56256

Moves the core Fling virtual collection components (VirtualRow, VirtualColumn,
VirtualCollectionView, and supporting files) from xplat/js/RKJSModules/Libraries/List/ into xplat/js/react-native-github/packages/react-native/src/private/components/virtualcollection/.

Updated 40 consumer files to import from 'react-native' instead of Haste and regenerated BUCK files

# Changelog: [Internal]

Key changes:
- Moved 10 files (Virtual, VirtualCollectionView, FlingConstants, VirtualColumn,
  VirtualColumnGenerator, VirtualRow, VirtualRowGenerator, FlingDebugItemOverlay,
  getScrollParent, isScrollableNode) into react-native-github
- Resolved all fb_internal imports (ReactNativeElement, ReadOnlyElement,
  ReactNativeFeatureFlags, createHiddenVirtualView) to use relative paths
  within src/private/
- Converted copyright headers from proprietary to MIT license
- Added public re-exports to react-native index.js and index.js.flow
  (VirtualColumn, VirtualRow, VirtualArray, VirtualCollection, VirtualItem,
  createVirtualCollectionView, VirtualColumnGenerator, getScrollParent,
  DEFAULT_INITIAL_NUM_TO_RENDER)

Reviewed By: mdvacca, lunaleaps

Differential Revision: D98039999

fbshipit-source-id: c0164f6933f19225fafc001cc0049d534032f2b2