added a DebuClassLoader::findFile() method to make the wrapping less invasive

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

i have classified it as a bug fix, since due to the wrapping it can break assumptions about the loaded class loader, so implementing this method at least doesnt break the assumption that ``findFile()`` is available.

actually i think we should also introduced a loader interface to reduce the duct typing