rustdoc: Parse intra-doc links using `rustc_parse`

[camelid]

This would simplify rustdoc's logic and reduce duplication with rustc. It would also make it easier to implement fully-qualified syntax.

Related: It would be good to avoid any path length–based resolution logic—e.g. we currently assume paths to enum variant fields are always at least three segments. But re-exports of variants break this assumption.

cc #t-rustdoc/meetings > 2026-01-12 @ 💬 @GuillaumeGomez
cc #150586 (comment)