Tracking Issue for Packages as (optional) namespaces

[syphar]

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

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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

[epage]

For past discussion, see https://rust-lang.zulipchat.com/#narrow/stream/356853-t-docs-rs/topic/Namespace.20packages.2C.20rustdoc.2C.20and.20docs.2Ers