fix: resource bundle accessor with swift concurrency + swift 6 mode (#1559)

In swift 6 mode + swift concurrency enabled the logic today cannot build
with the following error:

```terminal
error: static property 'module' is not concurrency-safe because it is nonisolated global shared mutable state
 9 |     /// This is a workaround for the provided module static var that doesn't always work.
10 |     /// https://forums.swift.org/t/unable-to-find-bundle-in-package-target-tests-when-package-depends-on-another-package-containing-resources-accessed-via-bundle-module/43974/5
11 |     static var module: Bundle = {
   |                |- error: static property 'module' is not concurrency-safe because it is nonisolated global shared mutable state
   |                |- note: convert 'module' to a 'let' constant to make 'Sendable' shared state immutable
   |                |- note: annotate 'module' with '@mainactor' if property should only be accessed from the main actor
   |                `- note: disable concurrency-safety checks if accesses are protected by an external synchronization mechanism
```

This change makes sure it can build

---------

Signed-off-by: Maxwell Elliott <maxwell@elliott.now>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: maxwellE

examples/resources_example/swift/Package.swift

@@ -8,6 +8,7 @@ let package = Package(
         .package(path: "../third_party/another_package_with_resources"),
         .package(path: "../third_party/app_lovin_sdk"),
         .package(path: "../third_party/package_with_resources"),
+        .package(path: "../third_party/package_with_resources_swift_6"),
         .package(url: "https://github.com/Iterable/swift-sdk", from: "6.5.11"),
         .package(url: "https://github.com/SDWebImage/SDWebImageSwiftUI.git", from: "3.1.3"),
         .package(url: "https://github.com/google/GoogleSignIn-iOS", from: "8.0.0"),

examples/resources_example/third_party/package_with_resources_swift_6/Package.swift

@@ -0,0 +1,37 @@
+// swift-tools-version:6.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "package-with-resources-swift-6",
+    defaultLocalization: "en",
+    platforms: [.iOS(.v13), .macOS(.v13)],
+    products: [
+        .library(name: "CoolUI", targets: ["CoolUI"]),
+    ],
+    targets: [
+        .target(
+            name: "CoolUI",
+            resources: [.process("Resources")],
+            swiftSettings: [
+                // Future Swift features
+                .enableUpcomingFeature("ExistentialAny"),
+                .enableUpcomingFeature("InternalImportsByDefault"),
+
+                .swiftLanguageMode(.v6)
+            ]
+        ),
+        .testTarget(
+            name: "CoolUITests",
+            dependencies: ["CoolUI"],
+            swiftSettings: [
+                // Future Swift features
+                .enableUpcomingFeature("ExistentialAny"),
+                .enableUpcomingFeature("InternalImportsByDefault"),
+
+                .swiftLanguageMode(.v6)
+            ]
+        )
+    ]
+)

examples/resources_example/third_party/package_with_resources_swift_6/Sources/CoolUI/CoolStuff.swift

@@ -0,0 +1,11 @@
+public import SwiftUI
+
+public enum CoolStuff {
+    public static func title() -> Text {
+        return Text("Title", bundle: .module)
+    }
+
+    public static func image() -> Image {
+        return Image("avatar", bundle: .module)
+    }
+}

examples/resources_example/third_party/package_with_resources_swift_6/Sources/CoolUI/Resources/Assets.xcassets/Contents.json

@@ -0,0 +1,6 @@
+{
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

examples/resources_example/third_party/package_with_resources_swift_6/Sources/CoolUI/Resources/Assets.xcassets/avatar.imageset/Contents.json

@@ -0,0 +1,15 @@
+{
+  "images" : [
+    {
+      "filename" : "ai_chatbot_avatar.pdf",
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  },
+  "properties" : {
+    "preserves-vector-representation" : true
+  }
+}

examples/resources_example/third_party/package_with_resources_swift_6/Sources/CoolUI/Resources/Assets.xcassets/avatar.imageset/ai_chatbot_avatar.pdf

@@ -0,0 +1 @@
+"Title" = "Hello, Avatar!";

examples/resources_example/third_party/package_with_resources_swift_6/Sources/CoolUI/Resources/Localization/en.lproj/Localizable.strings

@@ -0,0 +1,41 @@
+import Foundation
+import XCTest
+
+@testable import CoolUI
+
+class BundleNameTest: XCTestCase {
+    func testBundleName() {
+        let bundle = Bundle.bundle(named: "package-with-resources_CoolUI")
+        XCTAssertNotNil(bundle)
+    }
+}
+
+private class BundleFinder {}
+
+extension Foundation.Bundle {
+    static func bundle(named bundleName: String) -> Bundle? {
+        let candidates = [
+            // Bundle should be present here when the package is linked into an App.
+            Bundle.main.resourceURL,
+            
+            // Bundle should be present here when the package is linked into a framework.
+            Bundle(for: BundleFinder.self).resourceURL,
+            
+            // For command-line tools.
+            Bundle.main.bundleURL,
+            
+            // Bundle should be present here when running previews from a different package (this is the path to "…/Debug-iphonesimulator/").
+            Bundle(for: BundleFinder.self).resourceURL?.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent(),
+            Bundle(for: BundleFinder.self).resourceURL?.deletingLastPathComponent().deletingLastPathComponent(),
+        ]
+        
+        for candidate in candidates {
+            let bundlePath = candidate?.appendingPathComponent(bundleName + ".bundle")
+            if let bundle = bundlePath.flatMap(Bundle.init(url:)) {
+                return bundle
+            }
+        }
+
+        return nil
+    }
+}

examples/resources_example/third_party/package_with_resources_swift_6/Tests/CoolUITests/BundleNameTest.swift

@@ -8,7 +8,7 @@ extension Foundation.Bundle {
 
     /// This is a workaround for the provided module static var that doesn't always work.
     /// https://forums.swift.org/t/unable-to-find-bundle-in-package-target-tests-when-package-depends-on-another-package-containing-resources-accessed-via-bundle-module/43974/5
-    static var module: Bundle = {
+    static let module: Bundle = {
         let bundleName = "{BUNDLE_NAME}"
         let localBundleName = "LocalPackages_{BUNDLE_NAME}"