This gets Variable and VariableProxy out of the business of worrying about
Interfaces.

At the same time, get rid of the notion of "module variables". In ES6, variables
that refer to modules will be simply be CONST bindings to module namespace
objects.

The only change in logic here is one more early error:
duplicate export names are now rejected.

BUG=v8:1569
LOG=n

Review URL: https://codereview.chromium.org/918373002

Cr-Commit-Position: refs/heads/master@{#26708}