[BUGFIX] Invalid class name to file path conversion

The conversion from a class name such as
tx_extensionname_domain_model_somename to its corresponding PHP file
does not take Extbase naming conventions for the file name. This yields
an wrongly-cased file name which will be validated anyway on case
insensitive file systems.

This wrong conversion may lead to a PHP fatal error
Cannot redeclare class VendorName\Extension\Domain\Model\Somename

Fixes: #55256
Releases: 6.2
Change-Id: Ibe512945ac01f53844f790bea9f0d0cf58518c54
Reviewed-on: https://review.typo3.org/26999
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Marcin Sągol
Reviewed-by: Stefan Neufeind
Tested-by: Marcin Sągol
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers