[B+C] Clean up Bukkit files. Fixes BUKKIT-5771

## The issue:

The current bukkit code doesn't always respect the contribution guidelines.

## Justification for this PR:

As an opensource project, bukkit needs contributing guidelines to be followed to prevent the code from getting a mess.
These contribution guidelines have not been applied to the code already in the repository (and maybe to some code added after).

Updating the code to respect them allows users to have good examples of what they are supposed to write to see their contribution accepted.

It might also be interresting to update the contribution guidelines to mention the way the imports are supposed to be grouped/separated and provide an example of nms modifications AND obc code using imports from more than just java/obc.

## PR Breakdown:

+ Import are not sorted alphabetically, grouped and separated by tld (as asked by @turt2live).
+ No trailing spaces remain.
+ New lines have been added to the files missing them.

## Relevant PR(s):

https://github.com/Bukkit/CraftBukkit/pull/1408

## JIRA Ticket:

[BUKKIT-5771](https://bukkit.atlassian.net/browse/BUKKIT-5771)