StringSchema incorrectly treated as a model

[blast-hardcheese]

Resolves #1502

Following @djm1329's investigation, StringSchema was using modelTypeAlias, which wasn't designed to handle primitive type aliases. This may have been a hold-over from before the object model was standardized in swagger-parser, or perhaps it's just been a bug forever and #1407 exposed it.

This'll be a bugfix, not a minor, but due to early-semver seeming to be incompatible with dependsOn, the only way to get the tests to run successfully currently is to use minor to disable the early-semver check.

[codecov]

Codecov Report

Merging #1503 (9b6a872) into master (1ea36ab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   84.19%   84.19%           
=======================================
  Files          92       92           
  Lines        5561     5563    +2     
  Branches      140      140           
=======================================
+ Hits         4682     4684    +2     
  Misses        879      879           

Impacted Files	Coverage Δ
...a/dev/guardrail/generators/ProtocolGenerator.scala	98.66% <100.00%> (+<0.01%)	⬆️

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[djm1329]

lgtm