we don't need access to all mirrors in the known universe: we can work with what we have; for instance:
https://pypi.python.org/pypi/collective.cover/1.1b1/json
what else we can add? we can use passing tests on Travis CI for different versions of a package as a proof of compatibility; for instance:
we can use Coveralls or Landscape information as proof of quality; for instance:
https://landscape.io/github/plone/plone.api
resuming: we use the information we have on hand: cold, objective and verifiable information, maybe in the form of badges.
you can add more criteria here.