Squish: Fix checkDebuggingLibrary()

Additionally:
* extracted iterateKits() for better re-use
* added parameter to iterateQtVersions() for easier usage
  when using iterateKits() as well

Change-Id: I8bec8ca8e1e37a233c121dbcfbf1c89104f79a7d
Reviewed-by: Robert Loehning <robert.loehning@digia.com>