This change is only about the source tree. We have tmpfiles.d/, modprobe.d/,
sysctl.d/, and sysusers.d/, but for historical reasons, rules/ didn't fit this
pattern. We also *install* it as rules.d/. Let's rename to be consistent.
When the joystick is integrated directly into the machine, knowing
that the device is internal allows us to disable attached functionality
when the device is not used or inaccessible.
For example, this allows disabling rumble and accelerometer on
flip-console-like devices like the GPD-XD.
