Applies function to each of the files named in system-name/s. If system-closure? is true, then function is applied to the files of the systems that this system depends on. If incude-pathname? is true, the the function is applied to complete pathnames; otherwise the function is applied only to the name+type of each file. If include-non-source? is true, then all files are included. If it is nil (the default), then only source files will be included.