This is a re-land of https://codereview.chromium.org/2393303002/ with
an additional call to DisposeModuleEmbedderData() added to fix lsan failures.

Unifies the approaches used for storing the specifier -> module mapping
and the module -> directory mapping, using std::unordered_maps for both
and storing them per-Context.

This requires adding a method to the v8::Module API to get a hash code
for a Module, but allows slimming down the API in return: gone are
SetEmbedderData/GetEmbedderData, along with the fourth argument
to ResolveModuleCallback.

Besides a simpler API, this allows d8 to get closer to the HTML loader,
which requires each Realm to have a persistent module map (though this
capability is not yet exercised by any tests).

BUG=v8:1569
TBR=neis@chromium.org,jochen@chromium.org

Review-Url: https://codereview.chromium.org/2405313002
Cr-Commit-Position: refs/heads/master@{#40186}