I've built a custom Swift module using Swift Package Manager. It's intended to be used on Linux with Vapor (hence the use of SwiftPM), but I want to build and test it within Xcode first (i.e., with a simple macOS app for the front-end). The module builds fine, but when I add it to my Xcode project (the test app), I get an "Undefined symbols for architecture x86_64" error followed by dozens of errors pointing to the module code. The module target is definitely being built for macOS, and definitely for x86_64. So what might be going wrong?