This removes the need for depending on a global site.pp, and depending on the
checked out apt module to be called 'apt' as well.
It pulls in stdlib via sub-modules as I don't have a better way to deal with
dependencies yet. It has pinned the revision for stdlib to 2.2.1 which is the
minimum required version for apt to work.
This commit adds test coverage for apt::ppa.
This test coverage is suficient to verify the
code changes that resolve the issue with
aptitude update not being called when ppas were
added (#10451).
