Shell::FetchModuleTree assumes that the module at file_name wasn't
already fetched. Shell::ExecuteModule is calling into
FetchModuleTree without checking if the module is already in the module
map, violating this assumption.

This change fixes this by having Shell::ExecuteModule check for the
existence of the module before calling into Shell::ExecuteModule, the
same way that Shell::DoHostImportModuleDynamically does.

Bug: v8:12530
Change-Id: Ia038cbd1715e85c9c92c4554fd486c657ef952e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3388130Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78636}