launchpad bazaar repos need client bzr configuration

In the latest revision of the charm (cs:~content-fetcher-charmers/content-fetcher-3), archive_location of type 'lp:<blah>' for bzr repositories do not work.
charmhelpers used to use python-bzrlib, but since this library isn't available on focal, it was replaced with plain `bzr` commands.
On a new unit, the bazaar client won't be configured (`bzr whoami`), and won't have a launchpad login configured (`bzr lp-login`). We may also not want to create a launchpad login for it (for instance with public repositories).