Make @providesModule handling opt-in

I was going through old React issues and found https://github.com/facebook/react/issues/1145, which was the impetus for this change.

This changes the current behavior from parsing `@providesModule` all the time and having it overrule anything, to making it opt-in. If we wanted to get really crazy we could pull this out and only do that parsing in React's jsx-internal.

React build still works (without the new option actually, I think that means there's more to do here) but the test case in the linked issue does the right thing now, and still does the broken thing when using `--use-relative-requires`