Restore uninstallation of dependencies

[HebaruSan]

Problem

As of #2561, if you try to remove a module that other installed modules depend on, you can't; the Apply button stays disabled and the status bar reports missing dependencies:

Before that pull request, this was allowed and would allow you to remove the module and mods depending on it. This isn't something we should break.

Background

MainModList.ComputeChangeSetFromModList creates multiple RelationshipResolvers for different purposes. First it creates one or more in a loop to resolve all provides relationships (even if there are no provides relationships involved):

CKAN/GUI/MainModList.cs

Lines 604 to 616 in 8b791df

	while (!handled_all_too_many_provides)
	{
	//Can't await in catch clause - doesn't seem to work in mono. Hence this flag
	TooManyModsProvideKraken kraken;
	try
	{
	new RelationshipResolver(
	modules_to_install,
	modules_to_remove,
	options, registry, version);
	handled_all_too_many_provides = true;
	continue;
	}

After that it find mods with unsatisfied dependencies and queues them up to be removed:

CKAN/GUI/MainModList.cs

Lines 634 to 640 in 8b791df

	foreach (var dependency in registry.FindReverseDependencies(modules_to_remove.Select(mod=>mod.identifier)))
	{
	//TODO This would be a good place to have a event that alters the row's graphics to show it will be removed
	CkanModule module_by_version = registry.GetModuleByVersion(installed_modules[dependency].identifier,
	installed_modules[dependency].version) ?? registry.InstalledModule(dependency).Module;
	changeSet.Add(new ModChange(new GUIMod(module_by_version, registry, version), GUIModChangeType.Remove, null));
	}

Then at the end it creates another RelationshipResolver to determine the actual change set:

CKAN/GUI/MainModList.cs

Lines 642 to 651 in 8b791df

	var resolver = new RelationshipResolver(
	modules_to_install,
	changeSet.Where(change => change.ChangeType.Equals(GUIModChangeType.Remove)).Select(m => m.Mod.ToModule()),
	options, registry, version);
	changeSet.UnionWith(
	resolver.ModList()
	.Select(m => new ModChange(new GUIMod(m, registry, version), GUIModChangeType.Install, resolver.ReasonFor(m))));
	return changeSet;

Cause

As of #2561, RelationshipResolver has a parameter for modules to uninstall. This allows us to specify a complete change set for evaluation, so that operations that depend on removal for their validity can succeed.

When that parameter was added, the provides loop of MainModList.ComputeChangeSetFromModList was updated to pass modules_to_remove, but this caused exceptions to be thrown if you were removing a dependency. This loop isn't designed to handle those exceptions, so they propagated out and messed everything up.

Changes

Now the loop to resolve provides relationships passes null instead of modules_to_remove. This will avoid raising exceptions for missing dependencies, and so the rest of the logic can run as expected. Other usages of RelationshipResolver still use work as before.

Fixes #2578.