Handle KSP-AVC krefs

[HebaruSan]

Motivation

ExtraPlanetaryLaunchpads's netkan currently uses a $kref property with a hard coded URL. This means that we have to update the netkan manually every time there's a new release.

This mod has a publicly hosted remote .version file at http://taniwha.org/~bill/EL.version

The KSP-AVC spec says:

• DOWNLOAD - Optional
  Web address where the latest version can be downloaded.
  This is only used from the remote version file.

In principle this should let us pick up new versions automatically, but currently that functionality doesn't exist.

Changes

Now Netkan will support this format:

    "$kref": "#/ckan/ksp-avc/http://taniwha.org/~bill/EL.version",

The Id part of the $kref gives a URL to check for a .version file, which contains a DOWNLOAD property that points to the latest version of the mod.

To accomplish this, a new AvcKrefTransformer class is added (the awkward name is because there's already an AvcTransformer which handles $vref, which needs to remain separate and is called too late to handle a $kref properly). The remaining AVC properties are added to AvcVersion, and used by the new transformer to set metadata properties. Setting the download property allows the remaining transformers to download the file and index it.

To set version and ksp_version / _min / _max, AvcTransformer's logic for this is split out into a public static function for sharing.

In testing, the above $kref generated a .ckan file for ExtraPlanetaryLaunchpads 6.0.0 that exactly matches the one already in the index.

Fixes #286. Fixes #1765.

[politas]

#2516 is merged - Can you update Netkan/Validators/KrefValidator.cs in this and also add an update to Spec.md for the new valid $kref? Actually, while you're updating Spec.md, the Curse Kref info is out of date, too, isn't it?

[HebaruSan]

Thanks for the reminder, updated the validator and the spec.