Exclude special dotfiles from Zarrs

[jwodder]

Closes #1146.

To do:

• Exclude dotfiles when uploading a Zarr
• Exclude dotfiles when downloading a Zarr
• Exclude dotfiles when determining whether a Zarr is empty
• Exclude dotfiles when determining whether a Zarr has exceeded the maximum depth
• Investigate expanding the exclusion to special dotfiles at all levels, rather than just at the root

[codecov]

Codecov Report

Base: 88.14% // Head: 88.28% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (0064071) compared to base (48a222f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1147      +/-   ##
==========================================
+ Coverage   88.14%   88.28%   +0.14%     
==========================================
  Files          73       73              
  Lines        8727     8800      +73     
==========================================
+ Hits         7692     7769      +77     
+ Misses       1035     1031       -4     

Flag	Coverage Δ
unittests	88.28% <100.00%> (+0.14%)	⬆️

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

Impacted Files	Coverage Δ
dandi/cli/tests/test_digest.py	100.00% <100.00%> (ø)
dandi/download.py	88.04% <100.00%> (+0.04%)	⬆️
dandi/files/_private.py	98.03% <100.00%> (+2.20%)	⬆️
dandi/files/zarr.py	88.03% <100.00%> (+1.17%)	⬆️
dandi/support/digests.py	100.00% <100.00%> (ø)
dandi/support/threaded_walk.py	93.10% <100.00%> (+0.24%)	⬆️
dandi/tests/test_files.py	100.00% <100.00%> (ø)
dandi/utils.py	82.31% <100.00%> (+0.10%)	⬆️

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[yarikoptic]

Thank you @jwodder ! I am not 100% sure about making it unconditionally yet, but for now should be good to go.