Prefer explicit features for optional deps. (#681)

Implicit features are slated to be removed in a future version
of Rust (2024 edition).

Fixing this exposed 2 instances where the wrong feature was being
checked for `serde` vs `serde-serialize`.

crates/rapier3d-urdf/Cargo.toml

@@ -13,7 +13,7 @@ license = "Apache-2.0"
 edition = "2021"
 
 [features]
-stl = ["rapier3d-stl"]
+stl = ["dep:rapier3d-stl"]
 
 [dependencies]
 log = "0.4"