Always use default plugin and search paths as fallback.
Always use default plugin and search paths as fallback.

This ensures that we look in at least one sensible place if users have
bogus settings.

Change-Id: Ib8a667582a97232d174a3ae18337d8480e3ef482
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

gatzka/qt-labs-qbs,Distrotech/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,villytiger/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,villytiger/qbs,Distrotech/qbs,qt-labs/qbs