HMR watcher should not call module resolver on watcher thread

[Jarred-Sumner]

onFileUpdate is not being thread safe. It needs to enqueue a task to the main thread. It should coalesce these. Maybe we can get away with sending hashes instead of the file path bytes. It will need to allocate probably regardless.

Remember that module resolution opens lots of file descriptors and parses stuff. Benchmarking making it multi threaded some time ago made the bundler slower due to increasing the file descriptor table size.