MapboxNavigation and MapboxCoreNavigation should be subspecs

[1ec5]

There should only be one podspec in this repository, not two, and it should contain two subspecs – one for the UI stuff and one for the non-UI stuff. That’ll makes it more efficient to install the entire navigation SDK via CocoaPods. Not clear whether both subspecs should be included by default.

I’d consider this a blocker for publishing the podspecs on CocoaPods.org, because there isn’t a real way to remove a pod once it’s up there.

/ref Project-OSRM/osrm-text-instructions.swift#15
/cc @bsudekum @frederoni

[1ec5]

I attempted to reimplement MapboxCoreNavigation as a subspec of MapboxNavigation in #140. Unfortunately, it doesn’t appear to be possible to reconcile CocoaPods’ subspec concept with the broader goal of keeping MapboxCoreNavigation and MapboxNavigation in separate frameworks (also needed for Carthage compatibility).

Having investigated this approach and decided against it, we should push the two podspecs to CocoaPods trunk as-is.