Fix missing commands in plugins' HelpMap

[roro1506HD]

SimpleHelpMap#initializeCommands() was called twice on server startup:

• Once at the end of CraftServer#enablePlugins(PluginLoadOrder) if the load order was POSTWORLD
• Once near the end of MinecraftServer#initPostWorld()

Bootstrap commands being registered on server boostrap meant they were already registered on the first initializeCommands call.
JavaPlugin commands on the other hand, specifically brigadier commands registered using the LifeCycle event manager, are only registered before the second initializeCommands call.

The issue is that the HelpMap was not cleared before the second call, and the second call could not register all plugins commands because the plugin was already there, and HelpMap topics works in a way that do not allow overrides.

This PR fixes the two issues mentionned above:

• Fixes initializeCommands being called twice on server startup
• Fixes onEnable brigadier commands not being shown in /help <plugin> commands

Fixes #13230

[Owen1212055]

I am a bit confused how this doesnt affect when bukkit/brig commands are loaded during POST WORLD stage. How is that handled?

[roro1506HD]

What are you refering to "this"? The issue or the fix?

[Owen1212055]

The fix

[roro1506HD]

The second initializeCommands is called after POSTWORLD plugins are initialized:

    protected void initPostWorld() {
        // ...
        this.server.enablePlugins(org.bukkit.plugin.PluginLoadOrder.POSTWORLD);
        // ...
        ((org.bukkit.craftbukkit.help.SimpleHelpMap) this.server.getHelpMap()).initializeCommands();
        // ...
    }