[BUGFIX] Correct fetching of extension form definitions

The `retrieveYamlFilesFromExtensionFolders` function returned
the full path to the form configuration as key and the filename
as value. The filename used to `loadMetaData` that resulted in
that the file could not be found because it expects to get the
full path here.

We are adapting retrieveYamlFilesFromExtensionFolders so
we only work on the values returned from that array to avoid
confusions or misshaps like this in the future.

Resolves: #101767
Releases: main
Change-Id: If1b58eae8da7fd5ac913a6a89cfd0aa889a200ef
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80709


Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>