The controller knew which modules were built against which base artifacts and used it only when asked
(build --on). A merge that rebuilt the runtime image left forty-two modules on the old image until
somebody asked, twice, by hand (hq issue 186). The merge now takes every module standing on what moved,
through every layer, into the same rebuild, in base order — the same rebuild the flag does, asked by
the merge that made it necessary.
jschoubben