Fix issues simplification

[statasaurus]

The issues were two fold,

• the code lists were just appearing as null case it was trying to match the code list type to the variable type
• non-unique names for the derivation_id of predecessor variables when read in via the spec_to_metacore

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 86.65%. Comparing base (94a2519) to head (7682693).

❗ Current head 7682693 differs from pull request most recent head e21eb69. Consider uploading reports for the commit e21eb69 to get more accurate results

Files	Patch %	Lines
R/spec_builder.R	76.47%	4 Missing ⚠️
R/metacore.R	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #100      +/-   ##
==========================================
- Coverage   86.75%   86.65%   -0.10%     
==========================================
  Files           7        7              
  Lines        1004     1019      +15     
==========================================
+ Hits          871      883      +12     
- Misses        133      136       +3     

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[mstackhouse]

The changes here add some new lines that aren't covered by unit tests, and it looks like it would be beneficial to have those lines covered because it's a new consideration and scenario. @statasaurus can you add tests before we merge?