ModuleDescriptor will end up holding the set of data described in the
spec as a "Module record". This introduces a little bit of confusion
with ModuleInfo, but I hope that'll become clearer over time.

Also removed the interface-printing flags. We probably want
Module-printing flags, but that can wait until we have more
Module-related structures.

BUG=v8:1569
LOG=n

Review URL: https://codereview.chromium.org/935723004

Cr-Commit-Position: refs/heads/master@{#26728}