Split the Package.swift files for CompileTests.

Provide Swift version specifics, ensure under v6 it uses v6 mode to
ensure things compile cleanly there.

CompileTests/InternalImportsByDefault/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.8
+// swift-tools-version: 6.0
 
 // Package.swift
 //
@@ -10,7 +10,6 @@
 
 import PackageDescription
 
-#if compiler(>=5.9)
 let package = Package(
     name: "CompileTests",
     dependencies: [
@@ -37,20 +36,6 @@ let package = Package(
                 .plugin(name: "SwiftProtobufPlugin", package: "swift-protobuf")
             ]
         )
-    ]
-)
-#else
-let package = Package(
-    name: "CompileTests",
-    targets: [
-        .executableTarget(
-            name: "InternalImportsByDefault",
-            exclude: [
-                "swift-protobuf-config.json",
-                "Protos/SomeProtoWithBytes.proto",
-                "Protos/ServiceOnly.proto",
-            ]
-        )
-    ]
+    ],
+    swiftLanguageModes: [.v6]
 )
-#endif

CompileTests/InternalImportsByDefault/Package@swift-5.8.swift

@@ -0,0 +1,25 @@
+// swift-tools-version: 5.8
+
+// Package.swift
+//
+// Copyright (c) 2024 Apple Inc. and the project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See LICENSE.txt for license information:
+// https://github.com/apple/swift-protobuf/blob/main/LICENSE.txt
+
+import PackageDescription
+
+let package = Package(
+    name: "CompileTests",
+    targets: [
+        .executableTarget(
+            name: "InternalImportsByDefault",
+            exclude: [
+                "swift-protobuf-config.json",
+                "Protos/SomeProtoWithBytes.proto",
+                "Protos/ServiceOnly.proto",
+            ]
+        )
+    ]
+)

CompileTests/InternalImportsByDefault/Package@swift-5.9.swift

@@ -0,0 +1,40 @@
+// swift-tools-version: 5.9
+
+// Package.swift
+//
+// Copyright (c) 2024 Apple Inc. and the project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See LICENSE.txt for license information:
+// https://github.com/apple/swift-protobuf/blob/main/LICENSE.txt
+
+import PackageDescription
+
+let package = Package(
+    name: "CompileTests",
+    dependencies: [
+        .package(path: "../..")
+    ],
+    targets: [
+        .executableTarget(
+            name: "InternalImportsByDefault",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf")
+            ],
+            exclude: [
+                "Protos/SomeProtoWithBytes.proto",
+                "Protos/ServiceOnly.proto",
+            ],
+            swiftSettings: [
+                .enableExperimentalFeature("InternalImportsByDefault"),
+                .enableExperimentalFeature("AccessLevelOnImport"),
+                // Enable warnings as errors so the build fails if warnings are
+                // present in generated code.
+                .unsafeFlags(["-warnings-as-errors"]),
+            ],
+            plugins: [
+                .plugin(name: "SwiftProtobufPlugin", package: "swift-protobuf")
+            ]
+        )
+    ]
+)

CompileTests/MultiModule/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.6
+// swift-tools-version: 6.0
 
 import PackageDescription
 
@@ -36,5 +36,6 @@ let package = Package(
                 .target(name: "ImportsAPublicly"),
             ]
         ),
-    ]
+    ],
+    swiftLanguageModes: [.v6]
 )

CompileTests/MultiModule/Package@swift-5.8.swift

@@ -0,0 +1,40 @@
+// swift-tools-version: 5.8
+
+import PackageDescription
+
+let package = Package(
+    name: "CompileTests",
+    dependencies: [
+        .package(name: "swift-protobuf", path: "../..")
+    ],
+    targets: [
+        .testTarget(
+            name: "Test1",
+            dependencies: ["ImportsAPublicly"]
+        ),
+        .testTarget(
+            name: "Test2",
+            dependencies: ["ImportsImportsAPublicly"]
+        ),
+        .target(
+            name: "ModuleA",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf")
+            ]
+        ),
+        .target(
+            name: "ImportsAPublicly",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf"),
+                .target(name: "ModuleA"),
+            ]
+        ),
+        .target(
+            name: "ImportsImportsAPublicly",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf"),
+                .target(name: "ImportsAPublicly"),
+            ]
+        ),
+    ]
+)

CompileTests/MultiModule/Package@swift-5.9.swift

@@ -0,0 +1,40 @@
+// swift-tools-version: 5.9
+
+import PackageDescription
+
+let package = Package(
+    name: "CompileTests",
+    dependencies: [
+        .package(name: "swift-protobuf", path: "../..")
+    ],
+    targets: [
+        .testTarget(
+            name: "Test1",
+            dependencies: ["ImportsAPublicly"]
+        ),
+        .testTarget(
+            name: "Test2",
+            dependencies: ["ImportsImportsAPublicly"]
+        ),
+        .target(
+            name: "ModuleA",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf")
+            ]
+        ),
+        .target(
+            name: "ImportsAPublicly",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf"),
+                .target(name: "ModuleA"),
+            ]
+        ),
+        .target(
+            name: "ImportsImportsAPublicly",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf"),
+                .target(name: "ImportsAPublicly"),
+            ]
+        ),
+    ]
+)