Modules: simplify logic around allocation of module internal variables

Modules: simplify logic around allocation of module internal variables

Since recursive modules are gone, only the top-level scope can have
module inner scopes. Rename Scope::AllocateModulesRecursively to
Scope::AllocateModules, and add test showing the module Variables
are still allocated appropriately in the top level scope.

BUG=v8:1569,v8:3940
LOG=n

Review URL: https://codereview.chromium.org/999893003

Cr-Commit-Position: refs/heads/master@{#27143}