Use `git-annex copy --from ... --to ...`

[jwodder]

Closes #325.

[codecov[bot]]

Codecov Report

Base: 76.49% // Head: 76.46% // Decreases project coverage by -0.03% :warning:

Coverage data is based on head (d06ffe6) compared to base (72c1d32). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## draft #336 +/- ## ========================================== - Coverage 76.49% 76.46% -0.03% ========================================== Files 14 14 Lines 2442 2439 -3 Branches 537 537 ========================================== - Hits 1868 1865 -3 Misses 426 426 Partials 148 148 ``` | [Impacted Files](https://codecov.io/gh/dandi/dandisets/pull/336?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [tools/backups2datalad/\_\_main\_\_.py](https://codecov.io/gh/dandi/dandisets/pull/336?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL19fbWFpbl9fLnB5) | `64.83% <100.00%> (-0.45%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi)

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[jwodder]

@yarikoptic Ping.

[yarikoptic]

I upgraded git-annex and datalad in dandisets conda-forge:

(dandisets) dandi@drogon:/mnt/backup/dandi/dandisets/000003$ datalad --version
datalad 0.18.2
(dandisets) dandi@drogon:/mnt/backup/dandi/dandisets/000003$ git annex version | head
git-annex version: 10.20230227-gb02b9cc

let's proceed