rustdoc: Display `crate::` as the name of the crate in intra-doc links

[camelid]

This would solve the issue described in #84716 (comment) and wouldn't require using extern crate.

A link like [`crate::collections::HashMap`] in std would be rendered as std::collections::HashMap, and would continue to link to HashMap's docs.

I don't think there are any use cases where a library author would want crate:: to show up in their docs.

cc #84721
cc @jyn514 @joshtriplett

[jyn514]

I think this might be helpful, but it doesn't replace those issues. For this to be useful you have to know ahead of time that

1. the name of the crate won't work
2. crate:: will automatically be replaced

And I think 1. is the stumbling block. Given how easy the workaround is, I don't think we should special case this.

[camelid]

Is extern crate self as std; the workaround you're referencing?

[jyn514]

Yes.

[camelid]

I think the issue with that is then people are going to have to add extern crate everywhere. If in order to not have verbose intra-doc links, you have to use this workaround, then what's the purpose of having crate::?

[joshtriplett]

I think this is an improvement, in that rendering crate::xyz as the_actual_crate_name::xyz is what people will generally want.

However, the issue motivating this was making reference to the_actual_crate_name::xyz in documentation, and expecting that to work, and being surprised when it generated an error even though the same reference in a doctest worked.