Auto merge of #37954 - eddyb:rustdoc-2, r=alexcrichton

Auto merge of #37954 - eddyb:rustdoc-2, r=alexcrichton

rustdoc: link to cross-crate sources directly.

Fixes #37684 by implementing proper support for getting the `Span` of definitions across crates.
In rustdoc this is used to generate direct links to the original source instead of fragile redirects.

This functionality could be expanded further for making error reporting code more uniform and seamless across crates, although at the moment there is no actual source to print, only file/line/column information.

Closes #37870 which is also "fixes" #37684 by throwing away the builtin macro docs from libcore.
After this lands, #37727 could be reverted, although it doesn't matter much either way.