Default install_to to GameData/Mods for KSP2

[HebaruSan]

Motivation

Currently all KSP2 mods install to BepInEx/plugins. However, once the official mod loader is completed, this is expected to change to GameData/Mods.

I was expecting to change this in a new client release when the official mod loader was released, but the modding ecosystem has already begun experimenting with using this path. See KSP-CKAN/KSP2-NetKAN#68 and KSP-CKAN/KSP2-NetKAN#70, and current versions of SpaceWarp supposedly already allow it, which means we are likely to see mods soon that require it to be installed properly.

Changes

• Now GameData/Mods can be used in install_to for KSP2, including with subfolder paths
• BepInEx/plugins can still be used for KSP2, but no longer with subfolder paths (no current mods use subfolder paths)
• Now the value of install_to for the default KSP2 install stanza (the one used when the install property is not present) is GameData/Mods
• The spec and schema are updated to reflect these changes

FYI to @KSP-CKAN/wranglers and @cheese3660.

Risks

• The default can only be changed safely if it's not being used. Add install stanzas to KSP2 mods using the default KSP2-NetKAN#72 addressed this by ensuring that no KSP2 mods currently use the default install stanza.
• The completed official mod loader may or may not actually use this path. If they decide to change it before release (e.g., if they correctly decide that GameData/Mods makes less sense than simply GameData), then we'll have to do this all over again with the new path.
• Quantum and SpaceWarp are both attempting to make their support for this folder work the same as they expect the official loader to work, but it's still likely that there will be problems and incompatibilities down the line. Users will ask, why doesn't this (Quantum-compatible) mod work with the official loader, or SpaceWarp, etc.? We will probably have to point them to the announcement:

  The KSP2 modding scene is brand new and still quite volatile (much like the game itself), with major incompatible changes occurring without warning. You probably will have problems! Please be patient with mod authors and CKAN maintainers and understand what you're getting into.

[cheese3660]

A few comments while I have cell service

1. Subfolder paths make no sense under GameData/Mods
2. Why is the default being changed at this point? Both Quantum and SpaceWarp state that this is experimental

[techman83]

I see no risk here. I note that the only mods without it set have been updated, and we can be explicit until such time things have settled. At which point it'll just be dropping a line from the metadata where relevant. LGTM!