Only satisfy the Build-Depends field for source builds
The build source job executes dpkg-buildpackage --source
to run the
clean target and then build the source package. Policy 7.7 says that
only the Build-Depends and Build-Conflicts fields must be satisfied when
the clean target is invoked, so do just that.
Since apt build-dep currently does not have a "--clean-only" option behaving like "--arch-only" and "--indep-only", I've used dpkg's perl API to reliably extract Build-Depends and Build-Conflicts from the control fields. This can be cleaned up after https://bugs.debian.org/1021342 is fixed.
Should improve #187 (closed).