This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()
It also makes it easier for us to organize things like the version
history feature.
Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.
Some of these were wrong (scoreboard manager), others are counter to
everything everyone expects (Locations world being null, which wasnt ever safe EVER)
others are just too noisy.
Replace some with Contract to get rid of the nullability constraint and go back to
the old days of IDE not considering it strictly one way or the other.
Also, stop requiring annotations on package-private.
Introduces the next Developer Perk for Paper-API: Your plugin isn't yellow anymore.
Also fixed random dupe code in ensureServerConversions that got mistakenly set in the update.
