Re #8018. Copy Third_Party python packages.

This copies all the python packages in the Third_Party/lib/mac64/Python
directory to the build (bin) directory and also into the Contents/MacOS
directory in the installed kit.
These locations are already in the python search path within MantidPlot.
Re #8018. Fix silly error.
Merge remote-tracking branch 'origin/feature/8018_mac_python_packages'