* does not work under -T taint mode
* if you do a "use Text::Balanced", you also get "Text::Balanced::ErrorMsg" and
  "Text::Balanced::Extractor", because they are defined in the same file. Should
  these be recorded as edges in the graph, too?
  (These should be a node-cluster, with the file name as "center" node and the
   others added below it like:

  +-----------+
  | Foo       |
  +...........+
  | Foo:Error |
  +...........+
  | Foo:Msg   |
  +-----------+

* it should also catch things like:

  package A; use B;
  require C;
  sub foo { require D; }
  BEGIN { require E; }

  e.g. find A => B, A => C, A => E, but NOT A => D.

  The current heuristics is a bit spotty. We could use PPI for this, albeit this
  would slow things down quite a bit. So optimize PPI.

* Edges should as title have the connection (A => B), and nodes a text like:
  "vX.YZ uses x, is used by y"

* implement --depth (max recursion level)
* implement --mode=(recurse|glob), remove --recurse, implement --use