debian: switch to using debhelper and dh_python2 to build debs This is a much larger commit than I'd like, but I honestly don't see a good way to break it up and leave things working. Summary: We now use debian/rules with debhelper to build our debs. This is much more standard, and means we use dh_python2 to do things like handle leaving .pyc files out of the built debs. The resulting package is split into mercurial and mercurial-common, with the former being the hg stub and all the native .sos, and the latter being basically everything else. builddeb and dockerdeb are updated to use the new system. The old way (using dpkg by hand) breaks with the above changes because debian/control no longer contains a version string (that's now guessed from the phony changelog.) Tests are updated to assert that the right files end up in the right debs.