rustdoc: cross-crate source links are one level lower.

Previously this was adding one-too-many `..`s to the path for the
`gotosrc=...` links for local crates. Also, the `root_path` already ends
in `/`s so a trailing / shouldn't be added after the root (some servers
treat `...//...` different to `.../...` including the one running
doc.rust-lang.org).
auto merge of #14458 : huonw/rust/rustdoc-cross-crate-link-path, r=alexcrichton

Previously this was adding one-too-many `..`s to the path for the
`gotosrc=...` links for local crates. Also, the `root_path` already ends
in `/`s so a trailing / shouldn't be added after the root (some servers
treat `...//...` different to `.../...` including the one running
doc.rust-lang.org).