message: Use feature macro to check for support of filesystem
  Rather than defaulting to std::experimental::filesystem in files.h a feature
  macro can be used to determine what stdfs uses.
diff: #include <vcpkg/base/expected.h>
  -#if defined(_WIN32)
  +#if defined(_WIN32) || defined(__cpp_lib_filesystem)
  #include <filesystem>
  #else
  #include <experimental/filesystem>
  namespace fs
  {
  +#ifdef __cpp_lib_filesystem
  + namespace stdfs = std::filesystem;
  +#else
  namespace stdfs = std::experimental::filesystem;
  +#endif
  using stdfs::copy_options;
  using stdfs::file_status;
  