lsp: Identify language servers by their configuration (#35270)

- **WIP: reorganize dispositions**
- **Introduce a LocalToolchainStore trait and use it for LspAdapter
methods**

Closes #35782
Closes #27331

Release Notes:

- Python: Improved propagation of a selected virtual environment into
the LSP configuration. This should the make all language-related
features such as Go to definition or Find all references more reliable.

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/languages/src/rust.rs

@@ -109,14 +109,10 @@ impl LspAdapter for RustLspAdapter {
         SERVER_NAME.clone()
     }
 
-    fn manifest_name(&self) -> Option<ManifestName> {
-        Some(SharedString::new_static("Cargo.toml").into())
-    }
-
     async fn check_if_user_installed(
         &self,
         delegate: &dyn LspAdapterDelegate,
-        _: Arc<dyn LanguageToolchainStore>,
+        _: Option<Toolchain>,
         _: &AsyncApp,
     ) -> Option<LanguageServerBinary> {
         let path = delegate.which("rust-analyzer".as_ref()).await?;