Use dispatch_semaphore for eager-main-queue module setup gate

Summary:
Replace the `std::mutex` + `std::condition_variable` + `std::shared_ptr<bool>` trio that gates the JS thread on main-queue module construction with a single `dispatch_semaphore_t`. The wait block is invoked exactly once (from `_loadScriptFromSource:`'s `beforeLoad` lambda), so single-shot semaphore semantics fit the contract directly — no condition predicate, no shared state captured by three independent `shared_ptr`s.

Net: 12 lines deleted, 1 captured object instead of 3, no behavior change.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D105953979

fbshipit-source-id: 149e959d04e8466589798ae38c68059db77ecd8b