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

hugolib/hugo_sites.go

@@ -440,7 +440,11 @@ func (h *HugoSites) createMissingPages() error {
 							key = s.PathSpec.MakePathSanitized(key)
 						}
 						for _, p := range taxonomyPages {
-							if p.sections[0] == plural && p.sections[1] == key {
+							// Some people may have /authors/MaxMustermann etc. as paths.
+							// p.sections contains the raw values from the file system.
+							// See https://github.com/gohugoio/hugo/issues/4238
+							singularKey := s.PathSpec.MakePathSanitized(p.sections[1])
+							if p.sections[0] == plural && singularKey == key {
 								foundTaxonomyPage = true
 								break
 							}