Xcode 10 Compatibility

[stephencelis]

It's a bit late for me to dig in further, but the following directive seems to be getting tripped up in Xcode 10.

#if SWIFT_PACKAGE
  import Operadics
#endif

Reproduce:

1. Run sudo xcode select --switch /Applications/Xcode-beta.app/ where Xcode-beta.app is the first Xcode 10 WWDC beta.
2. Run swift package generate-xcodeproj against a project using Swiftx, e.g. https://github.com/bkase/DoctorPretty
3. Try to build the project.

It'll fail because Operadics isn't being imported, I assume because #if SWIFT_PACKAGE is false.

[stephencelis]

This may be an SPM issue, filed here: https://bugs.swift.org/browse/SR-7912