Resolves #6076 

I've taken the JSON version of the simple API and converted it into a
`LinkSource` so that the package-finding logic in the `PyPiRepository`
is very similar to - but annoyingly not quite the same as! - the
`LegacyRepository`.

I've also taken the opportunity to refactor the `LegacyRepository` ever
so slightly to emphasise that similarity. I think I've probably fixed a
small bug re caching and pre-releases: previously the processing for
ignored pre-releases was skipped when reading from the cache.

I believe this change will tend to be a modest performance hit. Eg
consider a package like `cryptography`, for which there are maybe a
couple of dozen downloads available at each release: to get the
available versions we now have to iterate over each of those files and
parse their names, rather than simply reading the answer.

However if the API that poetry currently uses is truly deprecated I see
little choice but to suck that up - or risk being in an awkward spot
when it is turned off. cf #5970, but worse.

Most of the changes are in the test fixtures:
- unversioned fixtures were generated from the existing fixtures: I
didn't want to download fresh data and start getting different answers
than the tests were expecting
- new versioned fixtures were downloaded fresh