debian: structure for building backports from a single branch
Source a makefile (when it exists) in debian/rules to assemble
a source package via:
* a debian.tar.gz tarball built from combining the contents of debian/
and debian/backports/$backport/debian/ using other details under
debian/backports/$backport
* an orig.tar.gz file (not generated by this makefile). This can (and
should) be the same for all backports.