Added RNA functions for adding and removing Driver Targets. Unfortunately, I couldn't do this by simply adding callbacks for the add/remove of the collection, as I've had to add to extra RNA functions to do that.
Example usage - driving Lamp Distance with Cube LocZ:
myOb= bpy.data.objects["Lamp"]
myOb.driver_add("data.distance")
# drivers is list of F-Curves that have driver data
drivers= myOb.animation_data.drivers
distDriver= drivers[0].driver
dtar= distDriver.add_target("ctrl1")
dtar.id_type= 'OBJECT'
dtar.id= bpy.data.objects["Cube"]
dtar.rna_path= "location"
dtar.array_index= 2
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