* rustdoc was doubly appending the file name to the path of where to
generate the source files, meanwhile, the [src] hyperlinks were not
* Added a flag to rustdoc::html::render::clean_srcpath to ignore the
last path component, i.e. the file name itself to prevent the issue
* This also avoids creating directories with the same name as source
files, and it makes sure the link to `main.css` is correct as well.
Fixes #23192