[BUGFIX] Fix race condition in module router

The module components (like the iframe wrapper) are
created asynchronously after their implementing module
(e.g. <typo3-iframe-module>) is loaded.

The router had a preparatory check that verified
that existing module elements are reused, but missed
to verifiy whether the element has been created in
parallel while the module components javascript module
was loaded. This happens when the module changes
while the module component is loaded for the first time.

That leads to a situation that the module componenent
was created twice, which rendered two module frames
into the backend.

Also resolve a long overdue todo while at it.

Resolves: #101851
Releases: main, 12.4, 11.5
Change-Id: I069f9e924d38b3abeb70c6d4e805d9c43a4f05e9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80893


Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>