Convert wall light to 3 pin plug
This is an interesting question because you are caught between different code sections. 

If you are attaching a fixture to a plug, the conductor must be described as a flexible cord or cable. That would be covered in NEC Article 400.12 "Uses not permitted" (1) As a substitute for a fixed wiring of a structure. (2) See @Harper (3) Where attached to building surfaces. So in short you can't use flexible cord as a permanent part of the structure. Most AHJ will tell you the minute you put one support on the flexible cable it becomes a permanent part of the structure.

So then you might say, "well I'll use NM and that is approved as permanent wiring." That would be fine but attachment plugs are only approved to be mounted to approved flexible cords and cables (NEC Sections 400.6 (A) and Table 400.4.). Also in NEC Article 334.12 (B)(4) Does not permit NM to be installed expose in wet or damp locations, and a garage is considered a damp location.

So then the only way you could use the method you are suggesting is to call it temporary wiring, but that would mean you couldn't permanently attach the fixtures to the wall. Also temporary installations have a time limit then you must take them down. 

Although this may not be the answer you want to here but you really can't connect lights the way you want to a receptacle and be code compliant. Unless that plug would be within about 6' of the fixture and you will have two plugs because you have two fixtures. 

I am not sure why you are insisting to unplug the fixtures, but have you considered a switch with a lock out cover instead.