Rollup merge of rust-lang#150586 - intra-doc-assoc-alias, r=GuillaumeGomez

rustdoc: Fix intra-doc link bugs involving type aliases and associated items

This PR:
- Add support for linking to fields of variants behind a type alias.
- Correctly resolve links to fields and variants behind a type alias to the alias's version of the docs.
- Refactor some intra-doc links code to make it simpler.
- Add tests for the status quo of linking to associated items behind aliases.

Future steps:
- Nail down the rules of when inherent and trait impls are inlined into an alias's docs, and when impls on the alias appear for the aliased type.
- Adjust the resolutions of intra-doc links, through aliases, to associated items based on these rules.

r? @GuillaumeGomez

Author: JonathanBrouwer