PluginLoader::loadApplet works both by passing a simple plugin
name and a full path of a plasmoid package.
However, when loading the optional C++ plugin, we have to look
in the standard plasmoid plugin path, so only consider the
tail of the path name, as is guaranteed that part is equal to
the plugin name.
BUG:359902
Change-Id: Ic7300b7c12d7693bcb5590e29de780109a8ff81c