Since we now have multiple classes inheriting from PluginSpec it makes sense
to rename PluginSpecImpl to the more accurate CppPluginSpec as it deals with
compiled plugins only.
Amends b39b192518
Change-Id: Icae9daed2be5a258a9918731a881c72cbe4f0fa4
Reviewed-by: Eike Ziller <eike.ziller@qt.io>