This prevents plugins sending wrong packets at the compiler level (or would, if we had a compiler). It's more robust than a getter for client/server and throwing an exception since a static analysis tool can detect faults created by sending wrong packets from the server. This is also used to deny service to dodgy clients which send wrong packets to the server to attack it.
this is in preparation for clientbound/serverbound packet separation. I did this already on another branch, but the changeset was dependent on a massive refactor to split apart packets and binarystream which i'm still not fully happy with.
