RFC: #3243

• rustc tracking issue: Tracking Issue for Packages as (optional) namespaces rust#122349
• cargo tracking issue: Tracking Issue for Packages as (optional) namespaces cargo#13576
• crates.io tracking issue: Tracking Issue for Packages as (optional) namespaces crates.io#8292

Allow a parent package to open its API namespace for other packages to participate

main points to be discussed / designed:

• allow :: in crate names, nothing should break
• allow navigation in the namespace (for example as a dropdown in the topbar, and/or as a section in the sidebar)
• perhaps build namespace crates together to be able to search them together, similar to when dependencies are built with rustdoc default