JuliaGraphs interop: export edges to (int,int) list

[tfiers]

i.e. provide

# Graphs.jl needs nodes to be integers
nodes = Dict(pkg => i for (i, pkg) in enumerate(packages))
node(pkg) = nodes[pkg]

sth like

packages, nodes, edges = PkgGraph.export_for_Graph(pkg)

or even sth that can be passed to one of the DiGraph constructors,
https://juliagraphs.org/Graphs.jl/dev/core_functions/simplegraphs/#Graphs.SimpleGraphs.SimpleDiGraph-Union{Tuple{Array{Graphs.SimpleGraphs.SimpleEdge{T},%201}},%20Tuple{T}}%20where%20T%3C:Integer

(no, we're not importing AbstractGraph: it's not defined in interface pkg, but rahter in Graphs.jl itself)

but yeah, it'd be sth as the above, (maybe with packages→labels?),
just that edges will be list of (int,int), not (str=>str)