Require usage of ::create() for packets

[dktapps]

::create() was introduced to solve the issue of forgetting to initialize packet fields, which led to many confusing issues for plugin devs.

In furtherance of this goal, we need to prevent the use of new WhateverPacket entirely, and require the usage of WhateverPacket::create() exclusively. This would likely be achieved by making DataPacket::__construct() private, and introducing a DataPacket::fromBuffer() : static static method.