move index wrapper to top level of build dir; simplify buildDir var logic in copySupplemental,  https://github.com/phetsims/phet-io-wrappers/issues/116
move index wrapper to top level of build dir; simplify buildDir var logic in copySupplemental,  https://github.com/phetsims/phet-io-wrappers/issues/116

phetsims/chipper,phetsims/chipper,phetsims/chipper