Make the web_asset_cache compile in single-threaded mode. (#23994)

# Objective

- `cargo t -p bevy_asset --features=http,https,web_asset_cache` fails to
compile.
- This is because we don't want to use `async_fs::File` in
single-threaded since that uses a blocking IO thread to read files. We
instead just do the blocking on the async thread since we're
single-threaded any way.

## Solution

- Use the `FileReader` from our sync_file_asset module if
`multi_threaded` is not enabled.

## Testing

- The tests pass without `multi_threaded` enabled.

Author: andriyDev

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

@@ -4,7 +4,7 @@ mod file_watcher;
 #[cfg(feature = "multi_threaded")]
 mod file_asset;
 #[cfg(not(feature = "multi_threaded"))]
-mod sync_file_asset;
+pub(crate) mod sync_file_asset;
 
 #[cfg(feature = "file_watcher")]
 pub use file_watcher::*;

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

@@ -16,7 +16,7 @@ use std::{
 
 use super::{FileAssetReader, FileAssetWriter};
 
-struct FileReader(File);
+pub(crate) struct FileReader(pub(crate) File);
 
 impl AsyncRead for FileReader {
     fn poll_read(

crates/bevy_asset/src/io/web.rs

@@ -243,7 +243,11 @@ mod web_asset_cache {
         let cache_path = PathBuf::from(CACHE_DIR).join(&filename);
 
         if cache_path.exists() {
+            #[cfg(feature = "multi_threaded")]
             let mut file = async_fs::File::open(&cache_path).await?;
+            #[cfg(not(feature = "multi_threaded"))]
+            let mut file =
+                crate::io::file::sync_file_asset::FileReader(std::fs::File::open(&cache_path)?);
             let mut buffer = Vec::new();
             file.read_to_end(&mut buffer).await?;
             Ok(Some(buffer))