Cross links to types in other crates within the registry are not linked

[calebfletcher]

The type SomeStruct in the image below is from another crate within the same registry, but it is not linked.

This should be fixed by adding --extern-html-root-url to the arguments passed to rustdoc.

[calebfletcher]

Blocked by rust-lang/cargo#11402 with a PR submitted in rust-lang/cargo#11403

[calebfletcher]

Current state is that linking to std and linking to crates.io crates works, but intra-registry links are broken until cargo can correctly produce the links for sparse registries.