How to switch yard lights without ending up with a really long circuit
If there is an existing power source where the lights will be installed you can use a lighting contactor to control the lights. Then then control wires will only need to carry maybe 20W-50W rather than 2kW your lights will draw. 

They generally need to be mounted in an electrical box to protect them from the elements. 

If the coil draws 40VA then 200m of AWG16 wire (400m round-trip) would be about 5 ohms and would drop less than 2V, which is more than acceptable.  

Check your local electrical codes for compliance requirements for the components and installation.