[rustdoc] Correctly handle rustc_allow_incoherent_impl on primitive methods

[GuillaumeGomez]

Needed for #162808.
First commit comes from #163133.

This is more of a hack than a proper fix as I don't think there is a better way to handle this. So in short:

• If a primitive incoherent impl method is implemented outside of core/alloc/std, we don't link to it.
• If in alloc/std, we link to std

r? @Urgau

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Urgau]

Did you try using the same trick previous the previous PR by using transitive_reexport_path? or does that also leads to the wrong path?

[GuillaumeGomez]

It leads to alloc, which doesn't have primitive pages. Hence the big ugly hack. :')

[Urgau]

Wait, what? []::sort is documented in core, but is defined in alloc!

Do you know why we do that?

Can't we document []::sort in alloc and avoid this ugly hack? I guess we can't do that bc there can only be one page per primitive?

[GuillaumeGomez]

Yep. Also, to_vec is present in std and not in core (and defined in alloc). Fun times. :)