[bug] Templates via classpath

[jimschubert]

At one point, we supported loading templates via class path, but there were a few conditions in code which removed this behavior.

You can test with https://github.com/jimschubert/template-classpath-example, which uses Bangers rather than the font used in the built-in template. The example demonstrates mustache templates loading relative template files rather than falling back to the embedded templates. In that repo:

gradle build publishToMavenLocal

In this repo:

mvn install
java -cp ~/.m2/repository/org/openapitools/samples/template-classpath-example/1.0-SNAPSHOT/template-classpath-example-1.0-SNAPSHOT.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar \
  org.openapitools.codegen.OpenAPIGenerator generate \
  -i https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml \
  -g html \
  -o template-example3 \
  -t templates/htmlDocs

This pulls templates/htmlDocs from the example's jar on the class path. That example results in this beautiful custom html documentation output:

Fixes #5136

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @OpenAPITools/generator-core-team

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