Fix path checking for FileWatcher for virtual workspace projects (#16958)

# Objective

Fixes #16879

## Solution

Moved the construction of the root path of the assets folder out of
`FileWatcher::new()` and into `source.rs`, as the path is checked there
with `path.exists()` and fails in certain configurations eg., virtual
workspaces.

## Testing

Applied fix to a private fork and tested against both standard project
setups and virtual workspaces. Works without issue on both. Have tested
under macOS and Arch Linux.

---------

Co-authored-by: JP Stringham <jp@bloomdigital.to>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_asset/src/io/file/file_watcher.rs

@@ -26,13 +26,13 @@ pub struct FileWatcher {
 
 impl FileWatcher {
     pub fn new(
-        root: PathBuf,
+        path: PathBuf,
         sender: Sender<AssetSourceEvent>,
         debounce_wait_time: Duration,
     ) -> Result<Self, notify::Error> {
-        let root = normalize_path(super::get_base_path().join(root).as_path());
+        let root = normalize_path(&path);
         let watcher = new_asset_event_debouncer(
-            root.clone(),
+            path.clone(),
             debounce_wait_time,
             FileEventHandler {
                 root,

crates/bevy_asset/src/io/source.rs

@@ -531,7 +531,7 @@ impl AssetSource {
                 not(target_os = "android")
             ))]
             {
-                let path = std::path::PathBuf::from(path.clone());
+                let path = super::file::get_base_path().join(path.clone());
                 if path.exists() {
                     Some(Box::new(
                         super::file::FileWatcher::new(