Experiment; WIP: BUGFIX: bugfix/use-md5paths-instead-classnames-to-flush-correctly

[mhsdesign]

resolves #3303

but this is just an experiment. I was under the impression that we cannot find out the namespace of a removed php file, but we can do a reverse lookup by taking all possible psr-4 autoloading configs into account and then doing the flush. See #3383

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• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[mhsdesign]

superseeded by #3383