If we do not encounter the load(qt_plugin) statement in the .pro file
but we do see the entry CONFIG+=plugin, treat the target as a regular
CMake library instead of treating it as a qt_plugin by default.
Change-Id: I67ad5c865a1a5ab691a6b0d86c2db4b686aa04dd
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>