PodToBUILD 0.25.2-38dd661

Handle nested sub-specs

Some podspecs use nested subspecs (eg
[`ReactCommon/turbomodule/core`](https://github.com/facebook/react-native/blob/0.61-stable/ReactCommon/ReactCommon.podspec#L58)).
Previously PodToBUILD hard coded a single level of subspec traversal,
only handling subspecs of the root spec. This changes it to handle
arbitrarily nested subspecs. The main change needed here was to pass
around a list of parent specs, rather than just a single nullable root
spec, and modifying usages of the root spec to either use the full
parents list or just the first element of the parents list, as
appropriate. It also expanded the source library target generation (eg
`objc_library`, `swift_library`) to be recursive.

Fixes #114