firebase-ios-sdk

Simplify/normlaize CMake platform libraries (#1605)

* Remove no-longer relevant comment

Nanopb headers now have distinct names so search order no longer
matters.

* cc_* rules now automatically filter out objc sources

* Combine executor and async queue into a single library

* Add cc_select

Adds a mechanism similar to blaze/bazel's select() for choosing an
implementation of a library.

This also makes the platform-neutral generic name for the library a
proper CMake library, which removes the need to keep libraries in
dependency order within CMakeLists.txt.

* Alphabetize rules