Support Cargo's "namespaced features"#55

Reference: - https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html - https://github.com/rust-lang/cargo/issues/5565 Example: ```toml [dependencies] ravif = { version = "0.6.3", optional = true } rgb = { version = "0.8.25", optional = true } [features] avif = ["dep:ravif", "dep:rgb"] ```