dockerdeb: rules to build a debian package using docker Currently only supports jessie (current stable), but other version should be trivial.

packaging: extract packagelib for common code from builddeb and buildrpm

builddeb: new script for building a deb package Future work will allow us to use docker to build debs. Right now this doesn't install any config files. I plan to do that as a followup, but getting something basic and working checked in seems like more of a priority than getting everything done in one big step. This also does not create a source deb yet. I haven't looked into that process. Note that this declares incompatibility with the `mercurial-common` package. It's typical for debian packages to be split between architecture-independent bits and native bits, meaning the python bits downstream live in mercurial-common and the c extension bits live in mercurial. We don't do that because we want to (ideally) give users a single deb file to install.