rustdoc should be able to reference libstd in intra-doc links in libcore

[clarfonthey]

This is something that came up in #112136.

Essentially, because we want to #[doc(inline)] the CString type in its parent module, we actually can't use hard-coded relative links and must use intra-doc links. However, we also can't do intra-doc links in libcore that reference libstd.

I think that especially since doc tests can reference libstd in libcore, we should also be able to intra-doc link libstd in libcore.

This also will probably enable us to get rid of the remaining hard-coded, non-intra-doc-links in libcore.

[clarfonthey]

@rustbot +T-rustdoc

EDIT: :(

[Urgau]

@rustbot labels +T-rustdoc +C-bug +A-intra-doc-links -needs-triage

[ehuss]

Is this issue much different from #74481?

[fmease]

Ah, I was just about to link to that issue ^^. Closing as duplicate of #74481.