`Psbt::sign` ok with `Xpriv` but fails with `HashMap<PublicKey, PrivateKey>`

If you see the API for `Psbt.sign()` you can pass (among other options) either:

- `HashMap<PublicKey, PrivateKey>`
- `Xpriv`

if you pass the `Xpriv` it signs ok, if you pass the `HashMap` it errors. Even if you derive `PublicKey` and `PrivateKey` straight from the `Xpriv`.

Do we need to add more stuff to the `Psbt` to sign with the `HashMap`?