Fix external link within file on export

[rly]

Motivation

Fix #846

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

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?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• 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: 90.90% and no project coverage change.

Comparison is base (f38e0b1) 87.81% compared to head (89e53c3) 87.81%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #847   +/-   ##
=======================================
  Coverage   87.81%   87.81%           
=======================================
  Files          44       44           
  Lines        8959     8962    +3     
  Branches     2698     2699    +1     
=======================================
+ Hits         7867     7870    +3     
  Misses        779      779           
  Partials      313      313           

Impacted Files	Coverage Δ
src/hdmf/backends/io.py	96.77% <75.00%> (+0.16%)	⬆️
src/hdmf/backends/hdf5/h5tools.py	82.47% <100.00%> (ø)

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