Remove available language only when it has loaded

This also ensures that, if you load the same language more than once,
a future that resolves to the language (or an error) is returned at
all times. Previously, we would only return it the first time the language
was loaded.

crates/zed/src/languages/go.rs

@@ -314,7 +314,7 @@ mod tests {
         let language = language(
             "go",
             tree_sitter_go::language(),
-            Some(Box::new(GoLspAdapter)),
+            Some(Arc::new(GoLspAdapter)),
         )
         .await;
 

crates/zed/src/languages/rust.rs

@@ -306,7 +306,7 @@ mod tests {
         let language = language(
             "rust",
             tree_sitter_rust::language(),
-            Some(Box::new(RustLspAdapter)),
+            Some(Arc::new(RustLspAdapter)),
         )
         .await;
         let grammar = language.grammar().unwrap();
@@ -392,7 +392,7 @@ mod tests {
         let language = language(
             "rust",
             tree_sitter_rust::language(),
-            Some(Box::new(RustLspAdapter)),
+            Some(Arc::new(RustLspAdapter)),
         )
         .await;
         let grammar = language.grammar().unwrap();