Yarn: Change PotionEffect and PotionEffectType to EffectStack and Effect

[Reoseah]

• remove Potion prefix from Effect classes, move them to effect package
• switch what is "base" name and what is "qualified" name
  Compare to similar classes for items: stateless thing is just Item, and wrapper around it is ItemStack.

Also, I personally prefer EffectDamageModifier to EffectDamage, despite it's a bit longer.

List of classes to cnange:

PotionEffect -> EffectStack
PotionEffectType -> Effect
PotionEffectTypeAbsorption -> EffectAbsorption
PotionEffectTypeDamage -> EffectDamageModifier
PotionEffectTypeHealthBoost -> EffectHealthBoost
PotionEffectTypeInstant -> EffectInstant
PotionEffectTypes -> Effects
PotionEffectUtil -> EffectUtils

[asiekierka]

There's another precedent: AttributeInstance. Likewise, we could use EffectInstance. ItemStack relates to the stackability of Items, which potion effects, well, don't.

[asiekierka]

In fact, official nomenclature refers to them as "mob effects". So MobEffect and MobEffectInstance it is.