dpkg build element

The specification (from Tristan) is: This should be a regular BuildElement derived element which will consist of a yaml file and a little bit of additional code.

The yaml will define how to build and install the software into the %{install-root} directory, using the debian/rules API, it assumes that the staged Source in question is a module that contains a debian subdirectory.

The python BuildElement derived class will additionally have to inspect some of the build results and populate the split-rules public data so that another dpkg deployment element will know the correct file manifests for the packages it creates.

This should come with a separate test case added to the .gitlab-ci.yml