(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.
This isn't specific to debian repos, which is why i put it in net. I'm open to other suggestions. Pkg::Repo might be fine, i don't have any other good ideas.