Instead of having a MODULE variable's index be 0 or 1, let it be the index of
its cell.  In this CL, we assign the indices but we continue to only use them to
distinguish imports from exports.  Actually using them to directly access the
cells will be done in a later CL.

R=adamk@chromium.org
BUG=v8:1569

Review-Url: https://codereview.chromium.org/2460233003
Cr-Commit-Position: refs/heads/master@{#40752}