Access to plan.json of v2-install

I'd be able to say

```base
cabal v2-install hlint --dry --something=this-dir
```

and to find `plan.json` somewhere inside `this-dir`.

---

My first idea is to make `--something` control temporary directory , and remove temporary file cleanup. That would be good enough approach.

i.e.

```
cabal v2-install hlint --dry --tmp-dir=this-dir --keep-temporary-files
```