CommandPacket -> Command

drop the packet, it's simpler.

CommandPacket is already a packet, and the generated Rust packets will
have Packet appended in some cases, which would lead to
CommandPacketPacket, which is not great.

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost SimpleHalTest
Change-Id: I5ad00f7c0f0d2dda83b3e1213145f0bd87c799e0