Filesystem watching crashes with symlinks pointing outside the asset folder

## Bevy version

0.8.0

## What you did

I have a symlink pointing to a directory containing some assets, that was located outside of Bevy's assets directory.

## What went wrong

As I modify a file in this directory, Bevy panics here: https://github.com/bevyengine/bevy/blob/main/crates/bevy_asset/src/io/file_asset_io.rs#L191 with `StripPrefixError`.

## Additional information

My attempt to debug this problem showed that (at least on macOS 12.5) the `notify` crate which Bevy uses for filesystem watching, returns an event with asset path `/Users/kon/repos/bomberman-of-the-hill/rounds/1/1.wasm` (the absolute path of the asset, after having followed the symlink) as a response to modifying this asset, whereas `asset_io.root_path` is set to `/Users/kon/repos/bomberman-of-the-hill/crates/bomber_game/assets/`. As you can see from this Playground snippet, this produces a `StripPrefixError`: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e4662b33f004e4f1a1f7d226a5dbe6c8

An interesting twist is that the asset would be reachable via path `/Users/kon/repos/bomberman-of-the-hill/crates/bomber_game/assets/rounds/1/1.wasm` and Bevy actually finds and uses this path when using the normal loading functions of `AssetServer` instead of filesystem watching. Of course, Bevy doesn't realize that the assets with differing paths are actually the same.