... via a comment in the API and a CHECK in
Isolate::RunHostInitializeImportMetaObjectCallback.

Also restructure things a little bit such that
this function really just runs the callback and
doesn't deal with module internals. Memoization
now happens in the SourceTextModule class.

Bug: v8:7044
Change-Id: I5b850ae629c3638c4b30dfdeaa996642a33d14dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190413Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67682}