[rustdoc] Fix source hyperlinks in docs

* 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.
* Added regression tests to ensure the rustdoc heirarchy of rendered
  source files remains consistent

Fixes #23192

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