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.
