Fix to remove access level modifier on imports for Swift 5

fix #870

refs #804

If you want without compiler conditional import, enable `InternalImportsByDefault` upcoming feature and remove `internal` from imports.

Author: kphrx

Sources/ArgumentParser/Utilities/Mutex.swift

@@ -10,10 +10,17 @@
 //===----------------------------------------------------------------------===//
 
 #if canImport(os)
+#if compiler(>=6.0)
 internal import os
 #if canImport(C.os.lock)
 internal import C.os.lock
 #endif
+#else
+import os
+#if canImport(C.os.lock)
+import C.os.lock
+#endif
+#endif
 #elseif canImport(Bionic)
 @preconcurrency import Bionic
 #elseif canImport(Glibc)