convert ppa:username/reponame to apt_line_uri

I think more convienient if ppa resource type accepts `ppa:launchpad-username/reponame` style.

Usually, PPA is denoted `ppa:launchpad-username/ppa-repository-name` style.

This PR makes that ppa resource type accepts both `ppa:username/reponame` and `username/reponame` style.