Validator fixes: fallback to our version, skip dotdirs on windows, etc

[TheChymera]

Miscellaneous but important improvements developed in parallel.
I will ping for review later, there are a few more.

[codecov]

Codecov Report

Merging #955 (1a6d835) into master (b08cae0) will increase coverage by 0.25%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #955      +/-   ##
==========================================
+ Coverage   87.33%   87.58%   +0.25%     
==========================================
  Files          65       65              
  Lines        8099     8119      +20     
==========================================
+ Hits         7073     7111      +38     
+ Misses       1026     1008      -18     

Flag	Coverage Δ
unittests	87.58% <85.71%> (+0.25%)	⬆️

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

Impacted Files	Coverage Δ
dandi/tests/test_bids_validator_xs.py	100.00% <ø> (ø)
dandi/bids_validator_xs.py	93.52% <85.71%> (+7.96%)	⬆️
dandi/files.py	78.66% <0.00%> (-1.13%)	⬇️
dandi/tests/test_files.py	100.00% <0.00%> (ø)
dandi/tests/test_download.py	100.00% <0.00%> (ø)
dandi/tests/fixtures.py	97.92% <0.00%> (+0.02%)	⬆️
dandi/download.py	87.24% <0.00%> (+0.29%)	⬆️

---

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[jwodder]

@TheChymera Is this PR finished/ready for review? If not, please mark it as a draft until it is done.

[TheChymera]

@jwodder should be good to go now. Sorry for the delay :(

[yarikoptic]

I adjusted title to be descriptive (will go into changelog), added labels including the one for release -- let's proceed. Thank you @TheChymera

[github-actions]

🚀 PR was released in 0.39.2 🚀