Enhancement: bzr source

From https://wiki.gnome.org/Projects/BuildStream/Roadmap/SourcePlugins A Bazaar source is needed to deal with bzr repositories. Since Flatpak builder supports this kind of source, it will be needed in order to automatically convert existing Flatpak JSON.

While a bazaar library (bzrlib) exists, and is used by bzr internally, it is only for python2, so is unsuitable, requiring us shell out to the bzr lib.