when reading in a .mag file. The routine was not checking for
whether a "use" entry in the file was the first one encountered
or not. The path is only ever given for the first use of any cell
def, so for any cell after the first, the path should have already
been resolved. This fix avoids lots of unnecessary error messages
when reading a file in a different directory. Also, because the
routine now checks for the first use in a file, any error messages
that do occur will only be displayed for the first use, not all of
them.