Move schema code to dandischema

[jwodder]

Closes #638.

This PR is pending a release of dandischema (now done).

[codecov]

Codecov Report

Merging #643 (8f657ab) into master (ac06349) will decrease coverage by 0.85%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
- Coverage   84.24%   83.39%   -0.86%     
==========================================
  Files          61       58       -3     
  Lines        5822     5196     -626     
==========================================
- Hits         4905     4333     -572     
+ Misses        917      863      -54     

Flag	Coverage Δ
unittests	83.39% <84.61%> (-0.86%)	⬇️

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

Impacted Files	Coverage Δ
dandi/cli/cmd_ls.py	66.46% <0.00%> (ø)
dandi/consts.py	100.00% <ø> (ø)
dandi/validate.py	53.57% <50.00%> (ø)
dandi/dandiset.py	84.21% <100.00%> (ø)
dandi/metadata.py	67.98% <100.00%> (-6.53%)	⬇️
dandi/tests/test_metadata.py	100.00% <100.00%> (ø)
dandi/tests/test_validate.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ac06349...8f657ab. Read the comment docs.

[yarikoptic]

What a meticulous amputation, nicely done @jwodder ;-) Haven't spotted any gotcha

[yarikoptic]

Let's proceed and hope for the bright future!