Not generating correct names for different output formats

I'm trying out Microbundle with https://github.com/reduxjs/redux-toolkit . For some reason, Microbundle seems to be ignoring the names that I've specified in the various `main/module/exports` fields, and is trying to reuse the the name from `main` for all output files.

Here's what I'm seeing. Currently, I've got `package.json` configured with these file names:

```js
"source": "src/index.ts",
"main": "dist/a.js",
"exports": "./dist/b.modern.js",
"module": "dist/c.module.js",
"unpkg": "dist/d.umd.js",
"types": "dist/typings.d.ts",
```

Microbundle reports this output:

```
Build "toolkit" to dist:
7.22 kB: a.js.gz
6.45 kB: a.js.br
6.06 kB: a.modern.js.gz
5.42 kB: a.modern.js.br
7.14 kB: a.module.js.gz
6.39 kB: a.module.js.br
7.31 kB: a.umd.js.gz
6.53 kB: a.umd.js.br
```

And the output files are:

```
dist/a.js
dist/a.modern.js
dist/a.module.js
dist/a.umd.js
```

Those are clearly not the file names I specified. Is there something obvious I'm missing here?

Start with the package.json fields and the src/index.ts entry point, then reproduce the build using the reported main, exports, module, unpkg, and types names. Compare the generated files in dist with the configured names and inspect the linked pull request for the current direction. Done means each output format uses its configured filename.

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