* do not add linkage scope to aliased exported symbols - this is
  not respected on macOS
* special-case `MachO.openPath` in `link.File.openPath` as on macOS
  we always link with zld
* redirect to `MachO.flushObject` when linking relocatable objects
  in MachO linker whereas move the entire linking logic into
  `MachO.flushModule`