chore(watch): simplify watch data synchronization (#9154)
### Description
Basic view of the futures involved with running `watch`
- The `events_fut` takes events from the package changes watcher and
folds them into a `ChangedPackages` struct. It then triggers a notify.
- The `run_fut` loops waiting for a notification and then performing a
run.
- Signal handler watching for `SIGINT`s
This PR cleans up how packages changes are synchronized between futures:
- Removes `RefCell` as `Mutex` already implies mutually exclusive access
the the underlying resource
- Remove `run_fut` holding onto the changed packages lock for the
entirety of it's run. This allows us to switch from `tokio::sync::Mutex`
to `std::sync::Mutex`. Which is suggested by the [`tokio::sync::Mutex`
docs](https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html#which-kind-of-mutex-should-you-use)
### Testing Instructions
`rustc` + 👀 . Quick gut check by running `turbo watch` and triggering
package changes
