7948: Delete `ContainerId` r=jonas-schievink a=jonas-schievink
Since block expressions containing items now have a `ModuleId`, there's no need to also treat `DefWithBodyId` as a potential item container. Since https://github.com/rust-analyzer/rust-analyzer/pull/7878, only the `ModuleId` variant of `ContainerId` was ever created, so just delete the thing and use `ModuleId` everywhere.
bors r+
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>