Use cargo's rustdoc-map instead to link to dependencies

[Nemo157]

Currently we're generating --extern-html-root-url flags to pass to rustdoc ourselves:

docs.rs/src/docbuilder/rustwide_builder.rs

Lines 521 to 529 in b527df8

	for dep in &cargo_metadata.root_dependencies() {
	rustdoc_flags.push("--extern-html-root-url".to_string());
	rustdoc_flags.push(format!(
	"{}=https://docs.rs/{}/{}",
	dep.name.replace("-", "_"),
	dep.name,
	dep.version
	));
	}

This has issues (#1166, #1165).

Cargo has an unstable rustdoc-map feature rust-lang/cargo#8296 intended to allow users to do a similar thing easily via a simple url template. We can likely use this instead of doing it manually.

[jyn514]

This needs cargo to link crates.io to docs.rs automatically before we can use it - right now you have to set a field in .cargo/config, but rustwide removes config before building the crate.