Fix: Packbeam support runtime: false & avoid duplicates

[petermm]

A new hello world project setup will yield a 40kb avm mix atomvm.packbeam.
Pr fixes that and yields a sub 1kb avm.

PR filters out packing exatomvm (and it's deps) - effectively same as if runtime: false was set for exatomvm, which seems more than correct.

Additionally adding deps like:

{:exatomvm, path: "../../ExAtomVM"},
{:pythonx, "~> 0.4.4", runtime: false},
{:igniter, "~> 0.5", runtime: false}

yields a 1.9mb avm, before this PR.

(the Enum.uniq() - brings that down to 1.2mb - create an |> IO.inspect before to see the duplicate modules)

PR, drills down from the top level and respects runtime: false - and yields a <1 kb avm.

Somewhat related info on the Mix.Dep.cached(): elixir-lang/elixir@af64fe7

fixes: #44 and enables the pythonx/igniter PRs.

[petermm]

removed the opinionated default of setting :exatomvm as runtime: false in all circumstances, so should be ready to go.