We were using $(SRCROOT) in the headerSearchPath to work around SwiftPM missing support for private headers and missing support for header-only interface framework dependencies.
Values in .headerSearchPath are relative to the target path so you can use ../ to go one level up. For example, .headerSearchPath("../GoogleUtilities/Logger/Private") should work for this case.
Ah. That looks like a bug in the generated Xcode project. Do you mind filing a JIRA for that? It should work if you open the package directly in Xcode.
Thanks, but opening the package directly in Xcode fails because of error: unexpected service error: build aborted due to an internal error: planningFailed("multiple configured targets of \'Firebase\' are being created for macOS")
that is described in detail here.