Distributed: do not import all of Darwin to implement locking facilities

At the same time, drop that dependency in the new build system, so that
we don't add an additional linkage for Darwin platforms compared to what
we are doing in the current build system.

Addresses rdar://158314427

Author: edymtt

Runtimes/Supplemental/Distributed/CMakeLists.txt

@@ -143,8 +143,7 @@ target_link_libraries(swiftDistributed PRIVATE
   swift_Concurrency
   swift_Builtin_float
   $<$<PLATFORM_ID:Android>:swiftAndroid>
-  $<$<PLATFORM_ID:Windows>:swiftWinSDK>
-  $<$<PLATFORM_ID:Darwin>:swiftDarwin>)
+  $<$<PLATFORM_ID:Windows>:swiftWinSDK>)
 
 install(TARGETS swiftDistributed
   EXPORT SwiftDistributedTargets

stdlib/public/Distributed/LocalTestingDistributedActorSystem.swift

@@ -12,8 +12,8 @@
 
 import Swift
 
-#if canImport(Darwin)
-import Darwin
+#if canImport(Darwin.os.lock)
+import Darwin.os.lock
 #elseif canImport(Glibc)
 import Glibc
 #elseif canImport(Musl)