rustdoc: improve refdef handling in the unresolved link lint
This commit takes advantage of a feature in pulldown-cmark that makes the list of link definitions available to the consuming application. It produces unresolved link warnings for refdefs that aren't used, and can now produce exact spans for the dest even when it has escapes.
Closes#133150 since this lint would have caught the mistake in that issue, and, along with https://github.com/rust-lang/rust-clippy/pull/13707, most mistakes in this class should produce a warning from one of them.
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