eliminate model package; converging on declcfg

[grokspawn]

Description of the change:
builds on #1986 but changes from a separate commit into a separate PR instead. This branches from it, so it includes #1986 content in this PR (and will get simpler if/once #1986 merges and is rebased).

Motivation for the change:

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[openshift-ci]

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[codecov]

Codecov Report

❌ Patch coverage is 69.62190% with 233 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.57%. Comparing base (e79d272) to head (8e66621).

Files with missing lines	Patch %	Lines
alpha/declcfg/api_conversions.go	35.22%	161 Missing and 10 partials ⚠️
alpha/declcfg/validate.go	94.42%	8 Missing and 6 partials ⚠️
alpha/action/render.go	61.29%	11 Missing and 1 partial ⚠️
alpha/action/list.go	90.09%	2 Missing and 8 partials ⚠️
pkg/cache/cache.go	65.21%	4 Missing and 4 partials ⚠️
pkg/cache/pkgs.go	82.60%	4 Missing and 4 partials ⚠️
alpha/declcfg/declcfg.go	66.66%	3 Missing and 3 partials ⚠️
alpha/action/init.go	91.30%	2 Missing ⚠️
alpha/template/substitutes/substitutes.go	50.00%	0 Missing and 1 partial ⚠️
pkg/lib/config/validate.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2025      +/-   ##
==========================================
- Coverage   58.81%   55.57%   -3.24%     
==========================================
  Files         141       97      -44     
  Lines       13430     8066    -5364     
==========================================
- Hits         7899     4483    -3416     
+ Misses       4322     3032    -1290     
+ Partials     1209      551     -658     

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