Auto merge of #92601 - camelid:more-intra-doc-cleanup, r=Manishearth

rustdoc: Remove the intra-doc links side channel

The side channel made the code much more complex and harder to
understand. It was added as a temporary workaround in
0c99d806eabd32a2ee2e6c71b400222b99c659e1, but it's no longer necessary.

The addition of `UrlFragment` in #92088 was the key to getting rid of
the side channel. The semantic information (rather than the strings that
used to be used for fragments) that is now captured by `UrlFragment` is
enough to obviate the side channel. An additional change had to be made
to `UrlFragment` in this PR to make this possible: it now records
`DefId`s rather than item names.

This PR also consolidates the checks for anchor conflicts into one place.

r? `@Manishearth`

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