[BUG] Breaking change in the resolution of relative imports between 4.3.0 and 4.3.1

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?
  4.3.0 and 4.3.1
• Have you search for related issues/PRs?

Description

My openapi description is decomposed into many files that reference each other via relative paths. The specification can be processed with v4.3.0 (and lower) successfully but the same fails with v4.3.1

openapi-generator version

The error occurs on v4.3.1. It is a regression since the same worked for v4.3.0 and lower.

OpenAPI declaration file content or url

Please check this repo for a minimal testcase since multiple files are involved:

https://github.com/Carsten-Leue/openapi-import-bug

Command line used for generation

successful command

java -jar openapi-generator-cli-4.3.0.jar generate -g openapi -i openapi/index.json -o out/4.3.0/

unsuccessful command

java -jar openapi-generator-cli-4.3.1.jar generate -g openapi -i openapi/index.json -o out/4.3.1/

Steps to reproduce

Checkout https://github.com/Carsten-Leue/openapi-import-bug and execute the run.bat script

Start with the minimal reproduction repository linked in the issue and run its run.bat script, comparing generation with openapi-generator-cli 4.3.0 and 4.3.1. Trace the relative-import resolution path responsible for the regression; done means the multi-file OpenAPI description processes successfully on 4.3.1 as it did on 4.3.0.