[rnpm] Support new import system in plugins

[grabbou]

This is iOS only.

We have to not addHeaderSearchPaths when a library opts-in to copy headers just like Facebook. That way, the are no longer needed.

That solution was raised by CodePush team as a possible solution and priority is to get it implemented before next release gets cut (or maybe even next set of cherry-picks lands).

[hramos]

We're cutting down on the number of outstanding issues, in order to allow us to focus. I'm closing this issue because it has been open for over 60 days with no activity. If you think it should still be opened let us know why. PRs are always welcome.