stinst: Add a pool look-up test for GNUPlot.GNUPlot like conditions.
stinst: Resolve GNUPlot.GNUPlot properly

Importing the Cairo and GNUPlot (probably other packages too) failed
because of GNUPlot.GNUPlot was looked up wrongly in the current
namespace. Add the current Namespace in front of all the other pools.