While running nxdbom on all software releases, it turned out that the
initial approach of listing all sections using slapos.recipe.build
without url= option was not practical, because there are tens of such
sections and it would be problematic if we need to change nxdbom every
time we add such a section.

Instead, just skip sections without an `url` option, keeping a small
heuristic to try not to miss usages: options that look like an URL are
flagged.