Default install stanza multi-game support, catch missing install_to

[HebaruSan]

Problem

In KSP-CKAN/NetKAN#8735, this metadata:

install:
  - find: GameData/InterstellarTechnologies
  - install_to: GameData

caused this error:

Error: 1459 [1] FATAL CKAN.NetKAN.Program (null) - Object reference not set to an instance of an object

Cause

install_to is required by the spec, and NetKAN.Validators.InstallValidator assumes install_to will be set. In this instance, it wasn't, because that YAML translates to:

  "install": [
    {
      "find": "GameData/InterstellarTechnologies"
    },
    {
      "install_to": "GameData"
    }
  ]

We should catch this explicitly rather than throwing a NRE.

Motivation

That PR's changes were essentially trying to make the default install stanza's behavior more explicit, so they've been set aside. However, in the process of explaining this I realized that the default install stanza is KSP1-specific: it hard-codes GameData. This would make default stanzas unusable for any future supported game that uses a different folder for mod data.

Changes

• Now if install_to isn't set, we throw an explicit error saying that
• Now the default install stanza uses the PrimaryModDirectoryRelative property of the module's game, which will be GameData for KSP1 and wherever mods are supposed to go instead for other games
• The spec is updated to hopefully be clearer about how the default install stanza works

[DasSkelett]

install_to is required by the spec

Interestingly, the client does work with a missing install_to in .ckan files, it fills in GameData when deserializing:

CKAN/Core/Types/ModuleInstallDescriptor.cs

Lines 40 to 42 in c087675

	[JsonProperty("install_to", DefaultValueHandling = DefaultValueHandling.IgnoreAndPopulate)]
	[DefaultValue("GameData")]
	public string install_to;

(Just tested it, works indeed)

Also related to this comment, the null check is indeed always false:

CKAN/Core/Types/ModuleInstallDescriptor.cs

Lines 75 to 80 in c087675

	// Make sure our install_to fields exists. We may be able to remove
	// this check now that we're doing better json-fu above.
	if (install_to == null)
	{
	throw new BadMetadataKraken(null, "Install stanzas must have an install_to");
	}

I guess we could keep it, as valid metadata should always have an install_to. Or we could also try to use game.PrimaryModDirectoryRelative, but this would require serialization context work etc.. Maybe we should just not populate it at all and throw the hard BadMetadataKraken.

[HebaruSan]

it fills in GameData when deserializing:
(Just tested it, works indeed)

Oh, I think that was done to save space in registry.json, since there were thousands of occurrences of install_to: GameData. So everyone's existing registry files have missing install_to properties, and we can't remove the defaulting now.

[DasSkelett]

Oh wow, you're right, they are indeed left out in registry.json. Didn't realize that before.