Ability to update repos without a game instance

[HebaruSan]

Background

The ckan update command retrieves fresh metadata from repositories, but currently it requires a game instance to work. Since #3904, repositories and game instances have been decoupled somewhat, so this should not be strictly necessary anymore.

Motivation

For https://github.com/KSP-CKAN/CKAN-ModInstaller, it would be nice to be able to pre-load the Docker image with metadata so a mod's build wouldn't have to wait for a ckan update command to run. Ideally we could do this without creating a temporary game instance during the docker build.

Changes

Now the ckan update command has several new flags:

• -g KSP -u URL [URL2 ...] will update the repos with the given URLs, using the -g parameter to determine the game to use
• -g KSP / -g KSP2 without -u will update the default repo for the given game (limit one per command)
• -f will force an update to be performed even if the repo's ETags haven't changed
• -l is now an alias for --list-changes

This will allow us to add these commands to the ModInstaller's Dockerfile to pre-load it with metadata:

ckan update -g KSP
ckan update -g KSP2