build.sh: explicitly dump extra debs
Set EXTRA_DEBS earlier, to make it available for dependency
checking. Add a loop over EXTRA_DEBS, running apt-cache show
on each of them for the dpkg dummy directory. This was necessary
to get build.sh working with extra packages that have not yet
been uploaded into the Debian archive. See #154 for more info.
This might be the wrong approach, but it got things working for me using @sylvestre 's recommended syntax.