[consensus] MOVEONLY: Move single-sig checking EvalScript code to EvalChecksig

Summary:
PR description:

This is in preparation for adding different signature verification rules,
specifically tapscript (BIP 342), which interprets opcode 0xac and 0xad
as Schnorr signature verifications.

The rationale for backporting this PR into ABC is that extracting a small piece of a huge function into its own function seems positive in itself, even if we don't plan to do the next steps described in the PR discussion.

This is a backport of Core PR18422

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8917