Provide schema validation results in package index project data

[per1234]

This allows the package index to be validated against its JSON schemas only once per project, with those results shared
between all the schema-based rules.

[codecov-commenter]

Codecov Report

Merging #193 (7a49563) into main (d7f669b) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #193      +/-   ##
==========================================
- Coverage   84.25%   84.23%   -0.02%     
==========================================
  Files          43       43              
  Lines        3207     3210       +3     
==========================================
+ Hits         2702     2704       +2     
- Misses        393      394       +1     
  Partials      112      112              

Flag	Coverage Δ
unit	84.23% <66.66%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
internal/project/projectdata/packageindex.go	94.44% <66.66%> (-1.64%)	⬇️

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