Ensure abstract classes are set to retrievable = false in ModelInfo.

[lukedegruchy]

Ensure abstract classes are set to retrievable = false in ModelInfo.

Description

Ensure StructureDefinitions with abstract result in the corresponding ModelInfo class having retrievable = false.

• Github Issue: https://github.com/cqframework/clinical_quality_language/issues/1392
• [X] I've read the contribution guidelines
• [X] Code compiles without errors
• [X] Tests are created / updated
• [X] Documentation is created / updated

By creating this PR you acknowledge that your contribution will be licensed under Apache 2.0

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 21.85%. Comparing base (6dd2f5c) to head (02d57ae). Report is 1 commits behind head on master.

Files	Patch %	Lines
...pencds/cqf/tooling/modelinfo/ClassInfoBuilder.java	0.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #533 +/- ## ============================================ - Coverage 21.85% 21.85% -0.01% Complexity 1673 1673 ============================================ Files 297 297 Lines 25390 25392 +2 Branches 4005 4004 -1 ============================================ Hits 5550 5550 - Misses 18920 18922 +2 Partials 920 920 ```

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