Speed up load with 2% by improving _register_watcher

I'm on the `main` branch of Panel trying to find performance improvements. I can see that `_register_watcher` shows up spending ~8% of the time in the profiling report.


My guess is that this can be improved 25% by looking up things more efficiently and testing `in` on sets or dictionaries instead of lists.

## Reproduce

```python
import panel as pn

N=10

pn.Column(
*[pn.Row(
*[pn.pane.Markdown("Hello World") for i in range(N)]
) for i in range(N)]
).servable()
```

Run

```bash
panel serve script.py --index script --admin --profiler pyinstrument
```

Open the app 5 times and inspect the performance report.