Rust doc `--extern-html-root-url` should support relative paths.

Hi!

I am using a bazel project and therefore am invoking rustdoc directly instead of through cargo rustdoc. Rustdoc has a --extern-html-root-url flag which allows you to specify external paths for crates.

Project setup:
I have a workspace of 2 crates:

• a
• b

a has public dependency of b and has functions which reference b's types in the public api.

If I invoke rustdoc by first building a's docs and then b the links for b in a's docs do not resolve, I am using the --extern-html-root-url flag to inform rustdoc to generate links for b relative to the output of a like this --extern-html-root-url b=/.

Ideally, if the path is relative rustdoc should make it relative to the output folder.

In a hierarchy where

• /a is a's docs
• /b is b's docs
• / is an index (maybe generated via --enable-index-page

Then if I add the flag --extern-html-root-url b=/ it should map to / relative to the index page.

Currently this is ignored.

in crate bang
references crate type_defs

build order: [bang, type_defs]

Start by reproducing the two-crate rustdoc build described in the issue, using --extern-html-root-url b=/ and checking the generated links. Trace rustdoc's handling of that flag and make relative paths resolve from the documentation output location; done means links from crate a to crate b resolve in the described hierarchy.