Enforcing sidedness for client package

[natanjunges]

Implements changes proposed in #3196, starting by the least impactful ones.

[modmuss50]

This looks like a good start likely should go into the next badly breaking Minecraft version.

Once this is ready we should also look into adding some tools to validate these packages. I can possibly look into moving yarn to use a split jar instead of a merged jar, should make this easier.

[natanjunges]

Where should I move the following classes to:

• net.minecraft.client.option.ChatVisibility should go under net.minecraft.network or net.minecraft.network.message?
• net.minecraft.client.render.BlockBreakingInfo should really go under net.minecraft.entity.player, or is there any other place where it fits better?
• net.minecraft.client.util.NetworkUtils should go under net.minecraft.utilor net.minecraft.network?