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.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information