Rollup merge of #144600 - Noratrieb:rustdoc-dep-info-paths, r=GuillaumeGomez

Ensure external paths passed via flags end up in rustdoc depinfo

rustdoc has many flags to pass external HTML/Markdown/CSS files that end up in the build. These need to be recorded in depinfo so that Cargo will rebuild the crate if they change.

Author: Zalathar