rustdoc: Don't load all extern crates unconditionally
Instead, only load the crates that are linked to with intra-doc links.
This doesn't help very much with any of rustdoc's fundamental issues
with freezing the resolver, but it at least fixes a stable-to-stable
regression, and makes the crate loading model somewhat more consistent
with rustc's. I tested and it unfortunately does not help at all with https://github.com/rust-lang/rust/pull/82496.
Closes https://github.com/rust-lang/rust/issues/68427. Let me know if you want me to open a separate issue for not freezing the resolver.
r? `@petrochenkov` cc `@eddyb` `@ollie27`
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