Relative import causes `ImportError` with `mil_usb_to_can` executables in Noetic

[cbrxyz]

In the mil_usb_to_can module, using relative imports causes the executable to fail. For example, this:

# mil_usb_to_can/driver.py
from .board import USBtoCANBoard
from .utils import USB2CANException

causes this ImportError when using rosrun mil_usb_to_can driver.py:

Traceback (most recent call last):
  File "/home/user/catkin_ws/src/mil/mil_common/drivers/mil_usb_to_can/mil_usb_to_can/driver.py", line 8, in <module>
    from .board import USBtoCANBoard
ImportError: attempted relative import with no known parent package

Currently, we will just use absolute imports. However, we need to explore why this issue occurs, and how we can move back to relative imports for increased brevity.