fix: exclude `basePath` in `findSourceMapURL` (#71719)
This PR fixes #71706.
When having an error client side and there is a `basePath` defined, the
basePath is being used to locate the sourcemap of the given file which
results in an error.
With these changes the `basePath` gets removed from the `filename`
passed to `findSourceMapURL`
If you have questions, feel free to ask!
