Introduces a new VariableLocation MODULE for variables that live in a
module's export table.  Scope analysis sets this for the approriate variables.
Not yet supported by any backend.

Also, treats all imports as CONST bindings (including namespace imports), rather
than having new special variable modes.

BUG=

Review-Url: https://codereview.chromium.org/2199283002
Cr-Commit-Position: refs/heads/master@{#38426}