BIP-0174 states that the transaction must be in the old serialization format (without witnesses).
BIP-0174 defines the derivation path being encoded as "<32-bit uint> <32-bit uint>*" with the asterisk meaning 0 to n times. Which in turn means that an empty path is valid, only the key fingerprint is mandatory.
In this commit, we move `btcutil` as well as its sub-module, the `psbt` package into the `btcd` repo itself.
