Function fetch_an_external_module_script 

pub(crate) fn fetch_an_external_module_script(
    cx: &mut JSContext,
    url: ServoUrl,
    global: &GlobalScope,
    options: ScriptFetchOptions,
    on_complete: impl FnOnce(&mut JSContext, Option<Rc<ModuleTree>>) + Clone + 'static,
)

https://html.spec.whatwg.org/multipage/#fetch-a-module-script-tree