Check redundant_explicit_links against generated URLs

[qaijuang]

Fixes #155458.

rustdoc::redundant_explicit_links checked explicit link destinations by resolving both the label and the destination as intra-doc paths. That misses links written as local rustdoc page URLs, such as struct.B.html, because those URLs do not appear in doc_link_resolutions.

This PR keeps the existing path-resolution check and falls back to comparing the explicit destination with the local href that rustdoc would generate for the resolved label.

[qaijuang]

cc @lolbinarycat

[rustbot]

r? @fmease

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[qaijuang]

I just noticed #156711, which seems related to broader rustdoc/compiler path-resolution work.

Does that tracking issue change the desired direction for this PR? If the preferred approach is to wait for the broader item-signature resolution mechanism, I’m happy to adjust/park this, but I’d appreciate guidance so I know whether this PR is still worth pursuing.

[fmease]

I've yet to review your PR (sorry about that), so I can't tell for certain. However, I'm inclined to believe that the feature described in that tracking issue is not relevant. Double-checking that later.

[rust-bors]

☔ The latest upstream changes (presumably #159046) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.