Code review follow-ups from: Plumb import assertions through SourceTextModuleDescriptor's ModuleRequestMap

Address a few bits of code review feedback that came in after landing
https://chromium-review.googlesource.com/c/v8/v8/+/2493060:

- Add ModuleRequest:kAssertionEntrySize and use in place of a numeric
  literal.
- Get rid of ModuleRequestLocation and separate module_request_positions
  FixedArray, and merge these into AstModuleRequest and
  v8::internal::ModuleRequest.

Change-Id: If6d628d29bfa6fbd9933c6cdaa706623128ccc5d
Bug: v8:10958
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2530478Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#71125}