USB dev_lowlevel gets incorrectly ignored when TinyUSB is unavailable

[lurch]

Whilst looking at the modifications to usb/CMakeLists.txt in #603 I realised that it currently says:

if (TARGET tinyusb_device)
    add_subdirectory(device)
else ()
    message("Skipping TinyUSB device examples as TinyUSB is unavailable")
endif ()

This means that the entirety of the usb/device/ directory (i.e. including usb/device/dev_lowlevel/) will get ignored if TinyUSB is unavailable, but according to https://github.com/raspberrypi/pico-examples/tree/develop?tab=readme-ov-file#low-level-example the dev_lowlevel example uses "direct access to the USB hardware (no TinyUSB)".

Perhaps after #603 has been merged, it would make sense to leave usb/device/dev_lowlevel/ in the same place, but move the TinyUSB device examples to usb/tinyusb/device/ ? 🤔 Alternatively, I guess we could modify usb/CMakeLists.txt to explicitly add_subdirectory(device/dev_lowlevel) in the else () branch shown above?