[Feature request] Use sourcemap files from imports

Using microbundle@0.15.1

In a multistage build (such as when using peggy), I'd like the final output map file to make use of map files declared by imported files. For example, if I run:

```bash
npx peggy --format es --output src/grammar.mjs --source-map src/grammar.mjs.map src/grammar.pegjs
# generates src/grammar.mjs and src/grammar.mjs.map
# src/grammar.mjs ends with `//# sourceMappingURL=grammar.mjs.map`
# src/index.mjs has `import { parse } from "src/grammar.mjs";`
npx microbundle src/index.mjs --compress=false --strict=true --sourcemap=true
```

I'd expect the resultant sourcemap to contain a reference to `../src/grammar.pegjs". Instead, it contains a reference to "../src/grammar.mjs".

使用 issue 中的两个 npx 命令，通过 src/grammar.pegjs、src/grammar.mjs、src/grammar.mjs.map 和 src/index.mjs 重现多阶段构建。跟踪 microbundle 如何处理导入的 sourceMappingURL；完成标准是生成的 sourcemap 引用 ../src/grammar.pegjs，而不是 ../src/grammar.mjs。

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