This patch gives the ability for the embedder to ask for the
module requests of a module, and to pass a ResolveCallback
into Module::Instantiate().

In d8, I've implemented a simple module_map that's used
along with this API to allow loading, compiling, instantiating,
and evaluating a whole tree of modules.

No path resolution is yet implemented, meaning that all
import paths are relative to whatever directory d8 runs
in. And no imports are linked to the exports of the
requested module.

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2351113004
Cr-Commit-Position: refs/heads/master@{#39569}