[backport#13932] Move PSBT UTXO fetching to a separate method

Summary:
https://github.com/bitcoin/bitcoin/pull/13932/commits/77542cf2a5f8abb97dd46f782c1b0199cc062033

Depends on D6044

This is a partial backport of Core PR13932

Test Plan: unfortunately no tests are included with this commit

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6045