rustdoc: Remove `ResolvedPath.did`
`ResolvedPath.did` was not actually the same as `.path.def_id()`. Instead,
`.did` referred to the `DefId` of the page to be used as a hyperlink target.
For example, a link to `Struct::method()` would use `Struct`'s `DefId` as its
`.did` field. This behavior is confusing, easy to accidentally misuse, and can
instead be obtained on-demand when computing hyperlink targets. It's also likely
part of the reason `kind_side_channel` exists. I'm currently working on some
experimental refactorings in `collect_intra_doc_links` that I believe require --
or at least benefit from -- removing `.did`.
r? `@jyn514`
862962b90e