This PR adds support for npm lockfile version 3, which drops the
"dependencies" key and uses "packages" instead. I've refactored the
lockfile parser to make the distinction between the versions explicit
rather than the implicit behaviour before. It _might_ be worth splitting
into separate files at some point, but the logic is so minimal that I
haven't done it.
Fixes#1203
Signed-off-by: Rob Cresswell <robcresswell@users.noreply.github.com>
