legacy repository: prefer JSON API to HTML API (#10672)

Motivation: some information (e.g. size and upload-time) that may be required in future for features like pylock.toml and minimumReleaseAge/exclude-newer is only available via JSON

Changes:
* prefer JSON in legacy repositories and fallback to HTML if JSON is not supported
* add support for JSON root pages
* add support for relative URLs in JSON pages
* add support for hashes in JSON pages
* extend legacy tests so that they are run with the HTML variant and the JSON variant
* harmonize HTML and JSON fixtures so that we get the same results in the tests