Allow plugins to depend on plugins outside of creator source tree

Do try to resolve plugin_dependencies.pri files in all directories
set by QTC_PLUGIN_DIRS. Also fail if a dependencies_pri file cannot
be resolved.

Change-Id: I2c14db832cef0fcc97a2f0351847c4969f5bd420
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>