Initial draft of compatibility with puppetlabs' apt module

This attempts to reproduce the _apt::source_ define that Puppetlabs' [apt](https://github.com/puppetlabs/puppetlabs-apt) module uses for the same purpose as example42's _apt::repository_, so that modules that expect Puppetlabs' apt module will continue to work.

Some features are ignored at this stage, see the TODO comments.

Please test before merging, as I've only had a chance to do **very** limited testing.

There may be other parts of the Puppetlabs apt module that we need to reproduce for compatibility, too..?