Compilation error when running swift build from the command line.

[randomeizer]

I have an SPM project which has CustomDump as a dependency. Just returned to it to update the library and am now getting this error message when running swift build on my project:

<my path>/.build/arm64-apple-macosx/debug/CustomDump.build/module.modulemap:2:12: error: header '<my path>/.build/arm64-apple-macosx/debug/CustomDump.build/CustomDump-Swift.h' not found
    header "<my path>/.build/arm64-apple-macosx/debug/CustomDump.build/CustomDump-Swift.h"
           ^
<my path>/Sources/OpenAIBitsTestHelpers/ClientHelpers.swift:1:8: error: could not build Objective-C module 'CustomDump'
import CustomDump
       ^
<my path>/.build/arm64-apple-macosx/debug/CustomDump.build/module.modulemap:2:12: error: header '<my path>/.build/arm64-apple-macosx/debug/CustomDump.build/CustomDump-Swift.h' not found
    header "<my path>/.build/arm64-apple-macosx/debug/CustomDump.build/CustomDump-Swift.h"
           ^
<my path>/Sources/OpenAIBitsTestHelpers/ClientHelpers.swift:1:8: error: could not build Objective-C module 'CustomDump'
import CustomDump

This is on a Mac Studio M1 Max.

This does compile correctly from within Xcode 14.2, after updating my Package.swift to 5.7 (was on 5.6 previously), so I'm thinking it may be a configuration error on my end?

Any thoughts on what I've done wrong?