Summary:
Adds the possibility to specify an array of files (group roots) that are used to bundle modules outside of the “startup section” into bigger groups by colocating their code.
A require call for any grouped module will load all modules in that group.
Files contained by multiple groups are deoptimized (i.e. bundled as individual script)
Reviewed By: martinbigio
Differential Revision: D3841780
fbshipit-source-id: 8d37782792efd66b5f557c7567489f68c9b229d8