hugolib: Fix handling of mixed-case taxonomy folders with content file

hugolib: Fix handling of mixed-case taxonomy folders with content file

* We match by path vs taxonomy to determine if we have a content page for that taxonomy
* The taxonomy name is (if `preserveTaxonomyNames` is not set) normalized to `maxmustermann` while you have the disk folder called `MaxMustermann`.
* This isn't a new issue, but I suspect most people will just name the folder `authors/maxmustermann` and it will just work.
* The inconsistent behaviour you see here is that you will end up with two pages with the same target filename, so it is a little random who will win.

This fixes that by also normalizing the taxonomy path when doing the comparison.

Fixes #4238