use splitpath instead of dirname

This is to try to fix a windows error, since I can't seem to create an
actual working build toolchain on Windows.

The error is:

    fileparse(): need a valid pathname at
    C:\strawberry5143\cpan\build\Beam-Wire-1.001-TQXaqw\blib\lib/Beam/Wire.pm
    line 35.
update build artifacts