Following up here in case someone else tries my same footsteps - getting the same details from a SwiftPM based build was a touch tricker, but thanks to help on Slack from @Aciid - doable. The invocation is:
swift build -Xswiftc=-enable-library-evolution --enable-parseable-module-interfaces
And the .swiftinterface files will be in the .build/x86_64-apple-macosx/debug directory