Warn on ignored symlinks to directories

[jwodder]

Closes #641.

[codecov]

Codecov Report

Merging #647 (6feb792) into master (edb49c6) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
- Coverage   83.32%   83.31%   -0.02%     
==========================================
  Files          58       58              
  Lines        5175     5177       +2     
==========================================
+ Hits         4312     4313       +1     
- Misses        863      864       +1     

Flag	Coverage Δ
unittests	83.31% <66.66%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
dandi/utils.py	80.39% <66.66%> (-0.20%)	⬇️

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[yarikoptic]

I think this should suffice for now. thank you @jwodder !