Detect link type for organize by linking from first src dir to dest dir

[jwodder]

Closes #1110.

[codecov]

Codecov Report

Base: 88.60% // Head: 88.59% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (1931c11) compared to base (e74c5ac).
Patch coverage: 88.88% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1115      +/-   ##
==========================================
- Coverage   88.60%   88.59%   -0.02%     
==========================================
  Files          76       76              
  Lines        9108     9117       +9     
==========================================
+ Hits         8070     8077       +7     
- Misses       1038     1040       +2     

Flag	Coverage Δ
unittests	88.59% <88.88%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/organize.py	82.81% <83.33%> (+0.06%)	⬆️
dandi/tests/test_organize.py	95.15% <100.00%> (+0.05%)	⬆️
dandi/files/bases.py	75.78% <0.00%> (-2.18%)	⬇️
dandi/support/threaded_walk.py	92.85% <0.00%> (-1.79%)	⬇️
dandi/upload.py	85.78% <0.00%> (-1.53%)	⬇️
dandi/metadata.py	87.02% <0.00%> (+1.01%)	⬆️
dandi/_version.py	45.96% <0.00%> (+1.75%)	⬆️

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[yarikoptic]

in principle AFAIK we do not even need link_test_file since we can simply use metadata[0]["path"] directly since we do load metadata records for regular paths too... but ok, let it be this way for now

[yarikoptic]

Thank you @jwodder and let's proceed!

[github-actions]

🚀 PR was released in 0.46.3 🚀