BUG: Fix Python site packages location detection.

This path is used to add WrapITK.pth so ITK's Python wrapping can be used from the build tree.

The current code only works when Slicer_USE_ITKPython is turned ON with an existing build tree. With a fresh build tree, Slicer's Python is not available at the initial configure. This patch delays the site package path detection to after ITK has been built, which is after Python has been built.