Use QTC_PLUGIN_DIRS also for generating the dependencies files

Use the same logic as in 27563451cb6286 to resolve out-of-source plugin
dependencies when generating the .pluginspec files.

Change-Id: I3a62527ff381aed0e83a7c37a50da403f0a6cece
Reviewed-by: Eike Ziller <eike.ziller@digia.com>