Remote swinfo files, sw vref warnings

[HebaruSan]

Motivations

• The SpaceWarp team has implemented a version_check URL property that works just like the version file URL property, but so far we don't support it
• If a mod has a swinfo.json file but its netkan has no $vref, there's no warning
• If a netkan has $vref: '#/ckan/space-warp' but the mod contains no swinfo.json file, there's no warning

Changes

• Now if swinfo.json contains version_check, we parse it as a URL, retrieve the designated page, and parse it as JSON. If this works and the local and remote version properties match, we use the remote file instead of the local file.
  • resources.remote-swinfo is set to this URL
  • ckan show and GUI display it
• Now if a mod has a swinfo.json file but its netkan has no $vref, netkan prints a warning
• Now if a netkan has $vref: '#/ckan/space-warp' but the mod contains no swinfo.json file, netkan prints a warning

Will self-review because this won't affect users until the full KSP2 stack is live.