Move dandi organize code to an organize() function

[jwodder]

Closes #897.

[codecov]

Codecov Report

Merging #902 (7272547) into master (49e33af) will decrease coverage by 0.04%.
The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
- Coverage   86.65%   86.61%   -0.05%     
==========================================
  Files          61       61              
  Lines        7435     7433       -2     
==========================================
- Hits         6443     6438       -5     
- Misses        992      995       +3     

Flag	Coverage Δ
unittests	86.61% <73.68%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
dandi/organize.py	82.23% <73.15%> (-4.76%)	⬇️
dandi/cli/cmd_organize.py	100.00% <100.00%> (+24.39%)	⬆️
dandi/conftest.py	52.94% <0.00%> (-47.06%)	⬇️
dandi/dandiarchive.py	84.42% <0.00%> (+2.04%)	⬆️

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[yarikoptic]

Thank you @jwodder.