runqueue: Fix setscene tasks not running

Currently, if you have hard dependencies between setscene tasks (like avahi on
base-passwd through useradd.bbclass), other dependencies may not be installed
even if these exist in sstate. For example, avahi -> expat -> pigz-native
(and avahi -> base-passwd) yet if you cleansstate base-passwd:

bitbake gzip-native:do_clean avahi:do_clean expat:do_clean pigz-native:do_clean base-passwd:do_cleansstate
bitbake avahi | tee

you will currently see pigz-native being rebuilt even though it was in
sstate. The fix for this is to continue to iterate dependency chains
around hard blocked dependencies as per this patch.

After this patch is applied, you will see pigz-native installed from sstate.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>