wallet.py nits: Refactor sweep_preparations
wallet.py nits: Refactor sweep_preparations

- Got rid of the public top-level function `append_utxos_to_inputs` in
wallet.py which *nothing* called except for `sweep_preparations`

- Instead, this is now a nested function inside `sweep_preparations`
thus making the code more readable & reducing namespace clutter

- Minor nit to not raise BaseException in wallet.py `sweep()` function
(this function is only called from the CLI/RPC interface)

fyookball/electrum,fyookball/electrum,fyookball/electrum