cran2deb: speed up dependency graph traversal. allow reverse arc traversal.

cran2deb: speed up dependency graph traversal.  allow reverse arc traversal.

A depends on B (r.dependency.of)
implies B is a prerequisite of A (r.prereq.of)

Note that r.prereq.of is rather slow; the default R data structures are
clearly intended for forward arc traversal only (A depends on B)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/cran2deb@44 edb9625f-4e0d-4859-8d74-9fd3b1da38cb