I have a Swift package like:

// swift-tools-version: 5.10
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "nmbr-icu",
    products: [
        .library(name: "nmbr-icu", targets: ["nmbr-icu"])
    ],
    targets: [
        // Main NMBR target, contains 1 ObjC++ file (h + mm)
        .target(name: "nmbr-icu",
                dependencies: ["ICU"]),
        // Target to build the ICU framework, an xcframework for the ICU C++ code
        .binaryTarget(name: "ICU",
                      path: "./ICU.xcframework"),
        // Test target
        .testTarget(name: "nmbrTests", dependencies: ["nmbr-icu"])
    ],
    cxxLanguageStandard: .cxx20
)

The project structure looks like:

When I open this package (in Xcode – by double-clicking the Package.swift file) I can build (CMD+B) and test (CMD+U). However, running swift build returns an error saying that one of the header files in my xcframework cannot be found:

NMBRFormatter.mm:11:10: fatal error: 'unicode/unistr.h' file not found
#include <unicode/unistr.h>

What am I missing? The repo is open source here: https://github.com/kerrmarin/nmbr-icu