This is a pile of non-blocking problems from #282 - I want to cut a release so Paper users (aka most users) aren't stuck waiting on me while I slowly learn how Gradle works.

• Replace script for Spigot installation with precompiled script plugin (or task) #290
• #293
  • Alternately, switch to mc-publish and let it discover versions? May still want to parse ranges ourselves.
• Fix CB versioned package name being specified in Paper module if possible 9f50de5
  • try to define in Spigot module so all Spigot versions are in the same place
• #289
• #287
  • Paper-only - Spigot will remain current Minecraft version only.
  • Should be able to expand support to 1.21.1 by abstracting placeholder system a bit further.
  • May be smarter to redesign internals a bit more with future backwards compatibility in mind rather than try to rework existing.
• #294
  • Should theoretically be possible by running some modified SpecialSource code prior to the current remap.
    • Modified SS is required; I have an issue open for the corresponding bug.
    • Should be "easy" to do by replacing exec task with task that depends on SS and just executes it.
  • There may be a different tool/fork that performs a similar function that can be used in place of SS. Paper's remapper works correctly, but I don't know if it is distributed separately from the userdev plugin, which I would like to avoid to ensure Spigot binary compatibility. Paper's remapper appears to execute SS with modified mappings at a glance. Paper uses tiny-remapper, which works well, but I'd prefer to try to work around the problem with SS instead for now.