perf(css): avoid constructing renderedModules (#19775)

Author: sapphi-red

packages/vite/src/node/plugins/css.ts

@@ -640,10 +640,19 @@ export function cssPostPlugin(config: ResolvedConfig): Plugin {
 
     async renderChunk(code, chunk, opts, meta) {
       let chunkCSS = ''
-      const renderedModules = Object.fromEntries(
-        Object.values(meta.chunks).flatMap((chunk) =>
-          Object.entries(chunk.modules),
-        ),
+      const renderedModules = new Proxy(
+        {} as Record<string, RenderedModule | undefined>,
+        {
+          get(_target, p) {
+            for (const name in meta.chunks) {
+              const modules = meta.chunks[name].modules
+              const module = modules[p as string]
+              if (module) {
+                return module
+              }
+            }
+          },
+        },
       )
       // the chunk is empty if it's a dynamic entry chunk that only contains a CSS import
       const isJsChunkEmpty = code === '' && !chunk.isEntry