message: Make SignatureExtractorChecker private to its own file
  It's only used here in DataFromTransaction
diff: @@ -243,6 +243,7 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
  return sigdata.complete;
  }
  +namespace {
  class SignatureExtractorChecker final : public BaseSignatureChecker
  {
  private:
  @@ -251,10 +252,7 @@ private:
  public:
  SignatureExtractorChecker(SignatureData& sigdata, BaseSignatureChecker& checker) : sigdata(sigdata), checker(checker) {}
  - bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override;
  -};
  -
  -bool SignatureExtractorChecker::CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const
  + bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override
  {
  if (checker.CheckSig(scriptSig, vchPubKey, scriptCode, sigversion)) {
  CPubKey pubkey(vchPubKey);
  @@ -263,9 +261,8 @@ bool SignatureExtractorChecker::CheckSig(const std::vector<unsigned char>& scrip
  }
  return false;
  }
  +};
  -namespace
  -{
  struct Stacks
  {
  std::vector<valtype> script;
  