rustdoc: Intra doc links from Deref/etc do not link to the same page

[jyn514]

A slight degradation in quality [from manual links] is that the #method.foo links would previously link to the same page on String's documentation, and now they will navigate to str.

Originally posted by @Manishearth in #74453.

[Manishearth]

More in general, if the thing being linked to exists on the same page, we should link to it on the same page.