Removes duplicate sweep tx code from commit resolver and delegates generation to UtxoSweeper in the sweep package.
This commit introduces a common interface for sweep inputs. It eliminates the type checking from UtxoSweeper. Also the formerly present Amount() getter is removed. It was redundant because this value is present in SignDesc().Output.Value as well.
Sweep txes are currently generated in multiple locations. Moving the sweep tx generation into a shared package will allow us to reuse that logic.