Allow specifying both project-wide and package-specific `clippy.toml` settings

For Ruma, I would like to have both project-wide and package-specific .clippy.toml settings:

• msrv (and import_rename once it's available) as a project-wide setting
• Possibly a project-specific msrv override
• disallowed_type project-specific setting to disallow HashMap / HashSet use within proc-macros

It seems like this is not currently possible, with the first .clippy.toml found in one of the parent directories of the package being checked being used for all settings. Maybe an alternative to the current filesystem hierarchy based config lookup¹ would be to only search the package dir² and the workspace root³ for clippy configuration and make any settings in the former overwrite settings from the latter?

¹ which AFAIK has proved problematic for Cargo, with some people wanting to move certain settings to Cargo.toml for better reproducability
² CARGO_MANIFEST_PATH
³ workspace_root in the cargo metadata --format-version 1 output