Adding PPAs could be easier

[Colin Watson]

It's possible to add PPAs using `package-repositories`, but it's unnecessarily cumbersome. At the moment it requires something like this:

        package-repositories:
            - type: apt
              formats: [deb]
              components: [main]
              suites: [bionic]
              url: https://ppa.launchpadcontent.net/launchpad/ppa/ubuntu
              # until lpcraft has `type: ppa` or support for specifying a key
              trusted: true

Roughly following https://snapcraft.io/docs/package-repositories, this could instead be:

        package-repositories:
            - type: apt
              formats: [deb]
              ppa: ppa:launchpad/ubuntu/ppa
              suites: [bionic]

(In this suggestion, I went for Launchpad's current archive reference syntax including the distribution rather than the old-style syntax that didn't; and I think it would make logical sense to include `suites`, although Snapcraft doesn't. `add-apt-repository` may be helpful in implementing this, either directly or as a reference.)