[BUG][JAVA] Regression "-Unable to load RELATIVE ref:"

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

This is a regression of the bug described here: https://github.com/swagger-api/swagger-parser/issues/505

Its working in 5.0.0 but is broken in 5.0.1 and also in the latest version 5.2.1

[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: ./openapi/model.yaml#/components/responses/FooResponse
Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 17
Errors: 
        -Unable to load RELATIVE ref: openapi/openapi/model.yaml path: /home/$user/$project
Warnings: 
        -Unable to load RELATIVE ref: openapi/openapi/model.yaml path: /home/$user/$project

        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:541)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:568)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:424)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

├── openapi
│   ├── model.yaml
└── openapi.yaml

...
$ref: ./openapi/model.yaml#/components/parameters/XXX
...

openapi-generator version

Broken versions probably everything > 5.0.0
I tested:

• 5.0.0 (working)
• 5.0.1 (not working)
• 5.2.1 (not working)

Generation Details

npx openapi-generator-cli generate \
    -i openapi.yaml \
    -g typescript-axios \
    -o ${OUTDIR} \
    --additional-properties=ngVersion=6.1.7,npmName=restClient,supportsES6=true,npmVersion=6.9.0,withInterfaces=true

Related issues/PRs

Here are all pull-requests between 5.0.0 and 5.0.1

https://github.com/OpenAPITools/openapi-generator/pulls?q=is%3Amerged+is%3Apr+milestone%3A5.0.1++

Suggest a fix

It could temporary be fixed by dereferencing everything into a single file.

Reproduce the regression with openapi.yaml, openapi/model.yaml, and the provided npx openapi-generator-cli command, comparing 5.0.0 with later versions. Start at CodegenConfigurator.java and the ModelUtils warning shown in the stack trace, then trace relative-reference loading. Done means the supplied relative $ref works again without the Unable to load RELATIVE ref error.