Simplify environment loading code (#40531)

This is a refactoring PR to simplify our environment loading code by:

- Getting rid of `EnvironmentErrorMessage` in favor of using
`anyhow::Result` everywhere, with a separate `mpsc` channel to
communicate statuses that will be shown in the activity indicator
- Inlining some functions that were only called once to reduce
indirection
- Removing the separate `direnv` module

Release Notes:

- N/A

crates/remote_server/src/headless_project.rs

@@ -94,7 +94,7 @@ impl HeadlessProject {
             store
         });
 
-        let environment = cx.new(|_| ProjectEnvironment::new(None));
+        let environment = cx.new(|cx| ProjectEnvironment::new(None, cx));
         let manifest_tree = ManifestTree::new(worktree_store.clone(), cx);
         let toolchain_store = cx.new(|cx| {
             ToolchainStore::local(