[BUG] openapi-generator-maven-plugin fails to read schema from the classpath

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

openapi-generator-maven-plugin fails to read schema from the classpath

openapi-generator version

3.3.4
4.0.x should have the same issue

OpenAPI declaration file content or url

n/a (see Steps to reproduce

Command line used for generation

n/a (see Steps to reproduce)

Steps to reproduce

I added modules/openapi-generator-maven-plugin/examples/multi-module in my fork repository.
Test fails in 3.3.4 (rev 4606f7711c), The problem is fixed in rev c8e8784c2d.

git clone https://github.com/vtitov/openapi-generator.git openapi-generator-multi-module && \
  cd openapi-generator-multi-module && \
  (git checkout 4606f7711c && \
      cd modules/openapi-generator-maven-plugin/examples/multi-module && \
      mvn clean package) || \
  (git checkout c8e8784c2d && \
      mvn clean install -pl '!modules/openapi-generator-gradle-plugin' -DskipTests -Dmaven.javadoc.skip && \
      cd modules/openapi-generator-maven-plugin/examples/multi-module && \
      mvn clean package)

Related issues/PRs

n/a

Suggest a fix

find a fix here:
https://github.com/vtitov/openapi-generator/tree/3.3.x

Start in modules/openapi-generator-maven-plugin/examples/multi-module and run the provided Maven reproduction at revision 4606f7711c. Compare it with revision c8e8784c2d and inspect the classpath schema-loading behavior in the Maven plugin. Done means mvn clean package succeeds for the multi-module example at the affected revision.