Fix export with link_data False not copying dsets

[rly]

Motivation

Fix #842

How to test the behavior?

pytest

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
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• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (fde784d) 87.79% compared to head (f7e79d7) 87.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #848      +/-   ##
==========================================
+ Coverage   87.79%   87.81%   +0.01%     
==========================================
  Files          44       44              
  Lines        8950     8959       +9     
  Branches     2695     2698       +3     
==========================================
+ Hits         7858     7867       +9     
  Misses        779      779              
  Partials      313      313              

Impacted Files	Coverage Δ
src/hdmf/backends/hdf5/h5tools.py	82.47% <100.00%> (+0.03%)	⬆️
src/hdmf/backends/io.py	96.61% <100.00%> (+0.24%)	⬆️
src/hdmf/build/manager.py	85.30% <100.00%> (+0.08%)	⬆️

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