Use std::cell::OnceCell and remove mitochondria dependency (#30207)

`OnceCell` is now part of the standard library and we'll be able to use
it once we upgrade rust. For now we can use the version that's shipped
behind a feature flag in rust. This removes a dependency on one crate.

components/script/lib.rs

@@ -4,6 +4,7 @@
 
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
+#![feature(once_cell)]
 #![feature(plugin)]
 #![feature(register_tool)]
 #![deny(unsafe_code)]