fix(plugin-legacy): group discovered polyfills by output

[bluwy]

Description

Since merging #16566, it caused CI to be flaky as in Rollup watch mode, the renderStart and renderChunk hooks are all running overlapping due to Rollup writing each output in parallel: https://github.com/rollup/rollup/blob/1a7da5ac529d543ad8fffd1cdfbf9a80040a1176/src/watch/watch.ts#L205-L216

In contrary to Vite where we write each output serially:

vite/packages/vite/src/node/build.ts

Lines 762 to 765 in 6236846

	const res: RollupOutput[] = []
	for (const output of normalizedOutputs) {
	res.push(await bundle[options.write ? 'write' : 'generate'](output))
	}

If we want to, we can fix watch mode to write serially manually, but I figured to fix this in plugin-legacy itself first. By grouping each output to its own map, we can handle the chunks in renderChunk in parallel.

I also updated the code to use the variables defensively to prevent issues like this in the future. I needed to hook into the Rollup watcher error event in order to get the error message:

  error: [vite:legacy-post-process] [BABEL] unknown file: Cannot read properties of undefined (reading 'legacy') (While processing: "base$0")
      at apply (file:///Users/bjorn/Work/oss/vite/packages/plugin-legacy/dist/index.mjs:454:72)
      at sync (/Users/bjorn/Work/oss/vite/node_modules/.pnpm/@babel+core@7.24.6/node_modules/@babel/core/src/gensync-utils/async.ts:31:25)
      at sync (/Users/bjorn/Work/oss/vite/node_modules/.pnpm/gensync@1.0.0-beta.2/node_modules/gensync/index.js:182:19)
      at factory (/Users/bjorn/Work/oss/vite/node_modules/.pnpm/gensync@1.0.0-beta.2/node_modules/gensync/index.js:210:24)
      at Generator.next (<anonymous>)
      at /Users/bjorn/Work/oss/vite/node_modules/.pnpm/@babel+core@7.24.6/node_modules/@babel/core/src/config/full.ts:271:23
      at Generator.next (<anonymous>)

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