Improve 'RemovedInSphinx90Warning's

Is your feature request related to a problem? Please describe.
I have a large documentation written in rst which is compiled into html using sphinx. I am getting

RemovedInSphinx90Warning: Sphinx 9 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.

warnings compiling with sphinx 8.
I tried to figure out which extension creates the issue so that I could file an issue, and this can be fixed for sphinx 9.
However, I was unable making sphinx point to the exact cause for this warning nor could I find the reason using grep on sphinx and the extensions sources.
Using the -W option to transform warnings to errors seems to have no effect on RemovedInSphinx90Warnings.

Describe the solution you'd like
sphinx should point to the exact reason for the warning so that extension developers have the early chance to fix things.

Describe alternatives you've considered

• Using grep on sphinx and the extension sources did not help.
• The warnings started to show up when updating sphinx, so there is no way to bisect the problem back the possible introduction of the problem.
• Just create a simple test project and test all extensions one by one to find the problematic one: Yes, I could do that, it's just 8 extensions in the project. It's annoying but doable. However, I think this is not a practical solution for the future as the problem will come up over and over and there should be a simple solution that helps to identify these kinds of problems simply. If there is no straight forward way to debug this, people won't care about it and won't fix it early and so I think this is an undesirable situation/solution.

Additional context
none

Start by reproducing the RemovedInSphinx90Warning with Sphinx 8 and the reported -W option, then trace how warnings from extensions are emitted and whether their source location is retained. The work is done when the warning identifies the exact originating extension or location and -W handles it consistently.