feat: graph supports external and built in modules

[kitsonk]

Closes #127

I think it is functionally complete, and I have all the existing tests running. I just need to add tests for the new stuff,

But basically it is a minor refactor for existing code which goes from:

LoadResponse {
  specifier,
  maybe_headers,
  content
}

To:

LoadResponse::Module {
  specifier,
  maybe_headers,
  content
}

And the new external and built-in would be:

LoadResponse::External(specifier);
LoadResponse::BuiltIn(specifier);

They will be represented in the graph as:

ModuleKind::External;
ModuleKind::BuiltIn;

[littledivy]

API wise LGTM! I don't think for eszips we need even external specifiers slots in the graph (?) but makes sense for other usecases.

[kitsonk]

I don't think for eszips we need even external specifiers slots in the graph

deno compile's usage of eszip may require it