7.1: Does not match the `legacyOutput` mode

In 7.1, #934 made declaration files for each format.

Here is my `tsup.config.ts` file:

```ts
export default defineConfig({
entry: ["src/index.ts"],
format: ["cjs", "esm"],
legacyOutput: true,
splitting: false,
sourcemap: false,
clean: true,
dts: true,
minify: true,
});
```

I'm using `legacyOutput` mode, which
> Outputs different formats to different folder instead of using different extensions

And here is the list of the files produced:

```
$ ls dist
esm index.d.mts index.d.ts index.js
$ ls dist/esm
index.js
```

Meaning, that the `esm` bundle is located in the corresponding folder.
However, its declaration file (`index.d.mts`) is not.

Moreover, it's impossible to use `onSuccess` callback for running `fs.rename` (in order to move `index.d.mts` into `esm` folder), since that callback is being fired BEFORE making declarations.

Could you please fix it by placing the declaration files into the same directories where you put the bundles?

**Also, in legacy mode, the declaration filename should be `.d.ts` instead of `.d.mts`.**

CC @andrewbranch

Reproduce the issue with the provided tsup.config.ts and inspect the generated dist contents, including the esm directory and declaration files. Trace the legacyOutput declaration-generation path and its interaction with onSuccess. Done means declarations are placed beside their corresponding bundles and legacy mode uses .d.ts filenames.

Written by the indexing model from the issue text.