on, then:
  1. existing installs
  2. wheels ordered via Wheel.support_index_min(self._supported_tags)
  3. source archives
If prefer_binary was set, then all wheels are sorted above sources.

Note: it was considered to embed this logic into the Link
      comparison operators, but then different sdist links
      with the same version, would have to be considered equal
z@ is not a supported wheel for this platform. It can't be sorted.r@