feat(schema): Move coordsystem.json rules out of rules.sidecars

[effigies]

We were mixing up JSON and sidecar rules, but conceptually, sidecar metadata belongs to the data file, not the JSON file itself. coordsystem.json is its own JSON file, and needs to be validated as its own thing, in the same way dataset_description.json is.

The upshot is that selectors should use json.<whatever> instead of sidecar.<whatever> to make conditional requirements, and it will be easiest to keep these entirely separate in the schema.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.10%. Comparing base (0de4c9c) to head (574f7f6).
Report is 297 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1892      +/-   ##
==========================================
- Coverage   88.22%   88.10%   -0.12%     
==========================================
  Files          16       16              
  Lines        1393     1396       +3     
==========================================
+ Hits         1229     1230       +1     
- Misses        164      166       +2     

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