Implement listing packages from deepdep

[ErdaradunGaztea]

Best implemented as an S3 method. The call would look like:

dd <- deepdep("versionsort")
packages(dd)

and would return a character vector of unique package names or a numeric vector of dependency levels with package names as vector names. Should this contain the central package (with level 0)? This would be rather easy to filter out if need arises.

I'm conflicted on the name, too, and the ones I consider are: packages(), unique(), and dependencies(). Probably the first one.