Critical fix to plugin handling: client modules adding certain plugin types...

Critical fix to plugin handling: client modules adding certain plugin types via the panels_include_directory() method were only half functioning.

Due to a foolish way of handling the module name in the first-round implementation of panels_include_directory(), module name information was not being properly recorded in plugins that needed it (layout plugins, esp.). The method of handling module name strings has been changed to resolve this problem.