Fixes#6626.
If either of the two fields FinalScriptSig or FinalScriptWitness is set
on an input of a PSBT then that results in most of the fields of that
input not to be serialized in the packet anymore, since the input is
considered to be complete.
But because a signer isn't supposed to set any of the Final* fields,
this was wrong from the beginning. Only the finalizer will set those
fields.
This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
