Reduce usage of exported.PacketI

[colin-axner]

Summary

Throughout the codebase we reference exported.PacketI in many place we could directly reference the concrete implementation. Ideally we would remove this interface entirely as it shouldn't be necessary. As a starting point we should update reference to exported.PacketI to be channeltypes.Packet. I believe the only packet this should stay as is, is in the application callbacks, for example WriteAcknowledgement (as this may be disruptive)

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[DimitrisJim]

follow up for WriteAcknowledgement #5774