validation: Make BlockManager::LookupBlockIndex const

Summary:
This concludes backport of core#21525
https://github.com/bitcoin/bitcoin/pull/21525/commits/7e8b5ee814b0b8c34acb20637ed4fc988ccba555

Test Plan: ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11507