Merge #3309

3309: Find cargo toml up the fs r=matklad a=not-much-io

Currently rust-analyzer will look for Cargo.toml in the root of the project and if failing that then go down the filesystem until root.

This unfortunately wouldn't work automatically with (what I imagine is) a fairly common project structure. As an example with multiple languages like:
```
js/
  ..
rust/
  Cargo.toml
  ...
```

Added this small change so rust-analyzer would glance one level up if not found in root or down the filesystem.

## Why not go deeper?

Could be problematic with large project vendored dependencies etc.

## Why not add a Cargo.toml manual setting option?

Loosely related and a good idea, however the convenience of having this automated also is hard to pass up. 

## Testing?

Build a binary with various logs and checked it in a project with such a structure:

```
[ERROR ra_project_model] find_cargo_toml()
[ERROR ra_project_model] find_cargo_toml_up_the_fs()
[ERROR ra_project_model] entities: ReadDir("/workspaces/my-project")
[ERROR ra_project_model] candidate: "/workspaces/my-project/rust/Cargo.toml", exists: true
```

## Edge Cases?

If you have multiple Cargo.toml files one level deeper AND not in the root, will get whatever comes first (order undefined), example:
```
crate1/
    Cargo.toml
crate2/
     Cargo.toml
... (no root Cargo.toml)
```

However this is quite unusual and wouldn't have worked before either. This is only resolvable via manually choosing.

Co-authored-by: nmio <kristo.koert@gmail.com>

crates/rust-analyzer/src/main_loop.rs

@@ -115,12 +115,15 @@ pub fn main_loop(
                     Ok(workspace) => loaded_workspaces.push(workspace),
                     Err(e) => {
                         log::error!("loading workspace failed: {:?}", e);
-                        if let Some(ra_project_model::CargoTomlNotFoundError(_)) = e.downcast_ref()
+
+                        if let Some(ra_project_model::CargoTomlNotFoundError { .. }) =
+                            e.downcast_ref()
                         {
                             if !feature_flags.get("notifications.cargo-toml-not-found") {
                                 continue;
                             }
                         }
+
                         show_message(
                             req::MessageType::Error,
                             format!("rust-analyzer failed to load workspace: {:?}", e),