Merge #17336: scripts: search for first block file for linearize-data with some block files pruned

317fb96de9c6257972f1213b4ef2c3fe87dde99f Add search for first blk file with pruned node (Rjected)

Pull request description:

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  When bitcoind is running in pruned mode, producing a hashlist with `./linearize-hashes.py linearize.cfg > hashlist.txt` and then executing `linearize-data.py linearize.cfg` will produce:
  ```
  Read 313001 hashes
  Input file /home/dan/.bitcoin/blocks/blk00000.dat
  Premature end of block data
  ```
  This happens because `linearize-data` starts by attempting to process `blk00000.dat` regardless of whether or not `blk00000.dat` actually exists - this may not be the case if working with a pruned node.
  This PR adds a function which finds the first block file that does exist, and calls that function when the `BlockDataCopier` is initialized.

  This is a refactor of #16431.

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ACKs for top commit:
  darosior:
    ACK 317fb96de9c6257972f1213b4ef2c3fe87dde99f
  laanwj:
    Code review ACK 317fb96de9c6257972f1213b4ef2c3fe87dde99f
  theStack:
    Code review ACK https://github.com/bitcoin/bitcoin/pull/17336/commits/317fb96de9c6257972f1213b4ef2c3fe87dde99f

Tree-SHA512: fc8014282df6cfe7b267e64db8ce7d82b86b758c302fbfea4a3c39b62d93512f5c2e31a0de4e9c5ec18fc0268c917f011257d37b45afaef6033eec90e4aa585f

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