SPRINGFIELD — A state law that allows prosecutors to appoint their own “special investigators” may seem simple enough.It says each state’s attorney in Illinois “shall” have the power to appoint one or more of them “to serve subpoenas, make return of process and conduct investigations which assist the [s]tate’s [a]ttorney in the performance of his duties.”But after one prosecutor used it to create a special drug interdiction unit, which made numerous arrests and seized …