Improvements to Darwin build ecosystem

I’d like to improve the build ecosystem on Darwin. Many Swift projects use cocoapods to manage cocoa modules; it is similar to bundler for Ruby gems. I’m relatively new to nix, so this is a call for collaboration.

There is currently a cocoapods derivation; however, it installs modules globally (in ‘Library/Caches/CocoaPods’). I suggest tooling that will sandbox the modules, perhaps that works like ‘bundlerEnv’, and ‘bundlerApp’.