[WIP, DNM] Foundation cross-import overlay

Package.swift

@@ -96,6 +96,7 @@ let package = Package(
       dependencies: [
         "Testing",
       ],
+      exclude: ["Foundation.swiftoverlay"],
       swiftSettings: .packageSettings
     ),
   ],

Sources/CMakeLists.txt

@@ -105,3 +105,4 @@ include(AvailabilityDefinitions)
 include(CompilerSettings)
 add_subdirectory(_TestingInternals)
 add_subdirectory(Testing)
+add_subdirectory(_Testing_Foundation)

Sources/_Testing_Foundation/CMakeLists.txt

@@ -0,0 +1,24 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2024 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+add_library(_Testing_Foundation SHARED
+  Events/Clock+Date.swift
+  ReexportTesting.swift)
+target_link_libraries(_Testing_Foundation PUBLIC
+  Testing)
+add_dependencies(_Testing_Foundation
+  Testing)
+target_compile_options(_Testing_Foundation PRIVATE
+  -enable-library-evolution
+  -emit-module-interface -emit-module-interface-path $<TARGET_PROPERTY:_Testing_Foundation,Swift_MODULE_DIRECTORY>/_Testing_Foundation.swiftinterface)
+
+get_swift_testing_install_lib_dir(SHARED overlays_destination_dir)
+install(FILES $<TARGET_PROPERTY:_Testing_Foundation,Swift_MODULE_DIRECTORY>/Foundation.swiftoverlay
+  DESTINATION "${overlays_destination_dir}/Testing.swiftcrossimport/Foundation.swiftoverlay")
+
+_swift_testing_install_target(_Testing_Foundation)

Sources/_Testing_Foundation/Foundation.swiftoverlay

@@ -0,0 +1,3 @@
+version: 1
+modules:
+  - name: _Testing_Foundation