[ReactNative] Smash child build files into parents

[jerrymarino]

This PR combines the child build files into the parents, which was introduced in a previous PR and is mainly used for pods like react native

Simply put:

• a parent podspec may glob paths included in a child dir
• a child may depend on a target defined in a parent podspec

These constraints aren't currently supported in Bazel, unfortunately.

This is covered in the build test, Examples/ChildPodspec which is also added onto the CI

As described in #134