ld: symbol(s) not found for archit… | Apple Developer Forums

My builds are failing after adding a Today Extension to my project which uses Swift Packages. I've posted an example here: https://github.com/emma-k-alexandra/SymbolsIssueExampleProject

Steps to reproduce project:

1. Create new iOS app

2. Add Today Extension to app

3. Add dependency https://github.com/emma-k-alexandra/WMATA.swift.git by Swift Packages menu

4. Add "import WMATA" to TodayViewController.swift, and add any WMATA.swift related code to viewDidLoad(). I added "print(MetroRail(key: "WMATA"))"

5. Run Today Extension target for iPhone 11 Pro Max.

I receive the following error:

Undefined symbols for architecture x86_64:
  "type metadata for WMATA.RailPrediction", referenced from:
      property wrapper backing initializer of Should_I_Run_Today_Extension.TrainView.prediction : WMATA.RailPrediction in TrainView.o
  "nominal type descriptor for WMATA.Line", referenced from:
      _symbolic Say_____G 5WMATA4LineO in Colors.o
  "WMATA.Station.lines.getter : [WMATA.Line]", referenced from:
      Should_I_Run_Today_Extension.gradientColors(WMATA.StationEntrance?) -> [SwiftUI.Color] in Colors.o
  "nominal type descriptor for WMATA.StationEntrance", referenced from:
      protocol conformance descriptor for WMATA.StationEntrance : Swift.Equatable in Should_I_Run_Today_Extension in WMATA.o
  "nominal type descriptor for WMATA.RailPrediction", referenced from:
      _symbolic _____ 5WMATA14RailPredictionV in WMATA.o
      protocol conformance descriptor for WMATA.RailPrediction : Swift.Identifiable in Should_I_Run_Today_Extension in WMATA.o
      _symbolic _____y_____G 7SwiftUI5StateV 5WMATA14RailPredictionV in TrainView.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I'm using Xcode 11.4, and can't figure out how to fix this linking issue.