Move all the hardware plugins from the `kaleidoscope::hardware` namespace to
`kaleidoscope::device`, in perparation for deeper changes to come later.
This is merely a restructuring, there are no functional changes. The one
breaking change is that `ATMegaKeyboard` moved too, and we do not provide any
backward compatibility there.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>