Fix embedded watcher to work with external crates (#11370)

# Objective

Tried using "embedded_watcher" feature and `embedded_asset!()` from
another crate. The assets embedded fine but were not "watched." The
problem appears to be that checking for the feature was done inside the
macro, so rather than checking if "embedded_watcher" was enabled for
bevy, it would check if it was enabled for the current crate.

## Solution

I extracted the checks for the "embedded_watcher" feature into its own
function called `watched_path()`. No external changes.

### Alternative Solution

An alternative fix would be to not do any feature checking in
`embedded_asset!()` or an extracted function and always send the
full_path to `insert_asset()` where it's promptly dropped when the
feature isn't turned on. That would be simpler.

```
    ($app: ident, $source_path: expr, $path: expr) => {{
        let mut embedded = $app
            .world
            .resource_mut::<$crate::io::embedded::EmbeddedAssetRegistry>();
        let path = $crate::embedded_path!($source_path, $path);
        //#[cfg(feature = "embedded_watcher")]
        let full_path = std::path::Path::new(file!()).parent().unwrap().join($path);
        //#[cfg(not(feature = "embedded_watcher"))]
        //let full_path = std::path::PathBuf::new();
        embedded.insert_asset(full_path, &path, include_bytes!($path));
    }};
```

## Changelog

> Fix embedded_watcher feature to work with external crates

crates/bevy_asset/src/io/embedded/mod.rs

@@ -199,14 +199,28 @@ macro_rules! embedded_asset {
             .world
             .resource_mut::<$crate::io::embedded::EmbeddedAssetRegistry>();
         let path = $crate::embedded_path!($source_path, $path);
-        #[cfg(feature = "embedded_watcher")]
-        let full_path = std::path::Path::new(file!()).parent().unwrap().join($path);
-        #[cfg(not(feature = "embedded_watcher"))]
-        let full_path = std::path::PathBuf::new();
-        embedded.insert_asset(full_path, &path, include_bytes!($path));
+        let watched_path = $crate::io::embedded::watched_path(file!(), $path);
+        embedded.insert_asset(watched_path, &path, include_bytes!($path));
     }};
 }
 
+/// Returns the path used by the watcher.
+#[doc(hidden)]
+#[cfg(feature = "embedded_watcher")]
+pub fn watched_path(source_file_path: &'static str, asset_path: &'static str) -> PathBuf {
+    PathBuf::from(source_file_path)
+        .parent()
+        .unwrap()
+        .join(asset_path)
+}
+
+/// Returns an empty PathBuf.
+#[doc(hidden)]
+#[cfg(not(feature = "embedded_watcher"))]
+pub fn watched_path(_source_file_path: &'static str, _asset_path: &'static str) -> PathBuf {
+    PathBuf::from("")
+}
+
 /// Loads an "internal" asset by embedding the string stored in the given `path_str` and associates it with the given handle.
 #[macro_export]
 macro_rules! load_internal_asset {