internal: better factoring for to_proto::completion

One source completion can produce up to two lsp completions.
Additionally, `preselct` and `sort_text` are global properties of the
whole set of completions, so the right granularity here is to convert
many completions.

As a side-benefit, we no loger allocate intermediate vec.

crates/rust-analyzer/src/lsp_ext.rs

@@ -499,3 +499,10 @@ pub enum WorkspaceSymbolSearchKind {
     OnlyTypes,
     AllSymbols,
 }
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct CompletionResolveData {
+    pub position: lsp_types::TextDocumentPositionParams,
+    pub full_import_path: String,
+    pub imported_name: String,
+}