Make the getSpentCoins() utility function public

Summary: So we can reuse it outside of the validation file. Move it to an appropriate place in the file and use an capitalized name for consistency. There is no change in behavior.

Test Plan:
With Chronik:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D18436