Update for Magma changes. Huge classloader changes

- Only cache Addon classes. Due to the "dynamic" nature of mods and how
  they tend to use ASM, we have no gurrantees on those classes. We need to
  look them up each and every time.
- Remove the need to "set" an addon on the AddonClassLoader. This was
  leftover from the old Spout plugin loading system this is loosely based
  upon and I devised a much cleaner approach.
- Handling all classloading from AddonClassLoader. This removes
   getClassByName from AddonManager
- No longer store a map of addon and their classes; the AddonClassLoader itself
  does this, we only store a set of those.
- Lookup classes from Forge first and not addons. This is a judgement call
  on my behalf as I feel addons will lookup classes from Forge before
  themselves. We do extend Forge after all.

Signed-off-by: Chris Sanders <zidane@outlook.com>