Make SignatureExtractorChecker private to its own file

Summary:
It's only used here in sign.cpp in DataFromTransaction(), so this enhances encapsulation.

This is a backport of Core PR13266

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8088