(CPR-54) Move deb_repos tasks into Pkg::Deb::Repo

To clear the way for CPR-52, having clean, parameterized interfaces is desireable.
This commit moves the content of the three tasks in deb_repos.rake into
lib/packaging/deb/repo.rb. This allows the rake tasks to be thin
wrappers around method calls and allows the methods to be tested at some
level.
I think if we were to go down a route, an erb template would make the most sense. But that's mostly because I have a well documented hatred of heredocs.