Improve Paper support

[Jikoo]

Apparently Paper has a setting to disable plugin remapping.

• Short-term:
  • [x] Check setting and make it clearer that this option is required to use OI.
  • [x] Inform Paper explicitly that OI is Spigot-mapped: https://docs.papermc.io/paper/dev/project-setup#spigot-mappings
• Long-term:
  • [ ] Separate Paper and Spigot versions.
  • This will allow us to keep jar files down and (hopefully) simplify the load process.
  • [ ] Stretch goal: look to re-use a common module and remap to all supported versions.
  • I believe Shopkeepers does this, so maybe they're a good example to learn from.

[Jikoo]

Per comment in restructure PR, stretch goals here will be tracked in the future planning project.