Dangerfile evaluation is single threaded

Right now it looks like evaluation of a DangerFile is basically single threaded, meaning if you want to do anything with the network you have to have dispatch locks. This is likely because we're not executing the app, but running the swift compiler int an faux-eval mode.

This might not work in the long run, and we might need to switch out to actually compiling using swift-pm and then executing that binary which handles the eval runtime.