This is a list of prefix, so if one of the builds fails
dependent ones will be rejected.
It would be nicer for a user perspective to specify that we want
to wait for the src.rpm of foo >= XX.YY to be available, with some
time limit, as it would get built if the original build fails but
get submitted again, but that would be much more complicated to
implement.
This simple solution allows a Youri::Submit action to submit build
of dependant packages while the upload is not finished yet. For
example this will allow to automatically rebuild kmod-* and
drakx-installer-images after a new kernel is uploaded.