rustdoc: Don't treat "super" specially for urls

This was just incorrectly handled before, the path component shouldn't be looked
at at all (we used absolute paths everywhere instead of relative to the current
module location).

Closes #9861
auto merge of #9916 : alexcrichton/rust/issue-9861, r=brson

This was just incorrectly handled before, the path component shouldn't be looked
at at all (we used absolute paths everywhere instead of relative to the current
module location).

Closes #9861