Share code between Apple platforms

[markvideon]

As per comments in #30, gamepads_ios and gamepads_darwin have a lot of shared functionality, at the moment they exist as entirely seperate packages.

As Luan mentioned, audioplayers could be used as a model for how to share code between these platforms.

There is also a proposal document from the Flutter team on the matter of sharing code between these platforms.

[spydon]

I think symlinks should be possible to use in packages now. :)