[BUG] Different Prefixes for Models and Operations Should Be Allowed

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?
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Description

When executing openapi-codegenerator with the command line option: --model-name-prefix=, this should only change the prefix of the models. Currently it changes both the model prefix and the operation prefix. This behavior prevents the use of external models if they are generated using openapi-codegenerator. I am currently generating all my models in a separate API spec and putting them into a library. I am then trying to generate a server using a different API spec that references my models API spec. I need to be able to change the models prefix to be different from my server prefix.

openapi-generator version

openapi-generator-cli-4.1.1
cpp-qt5-qhttpengine-server/cpp-qt5-client

OpenAPI declaration file content or url

Command line used for generation

java -Dapis -DsupportingFiles -DdebugModels -DdebugOperations -jar ${OPENAPI_JAR}/openapi-codegen.jar generate --enable-post-process-file --input-spec $1 --generator-name cpp-qt5-qhttpengine-server --template-dir /space/data/Repositories/MustacheFiles/ForQHTTPEngine --output $2 --model-name-prefix=MniPars

NOTE: By using DdebugModels and DdebugOperations and I can see that both the 'prefix' and 'modelNamePrefix' are both set by the model-name-prefix command line argument.

Steps to reproduce

SnapShot of the output for the above command:

Related issues/PRs

Suggest a fix

Users should have the capability to change both prefixes differently. This will allow the use of external models. By using the -DdebugModels and -DdebugOperations, the two tags used to apply the prefix are: 'prefix' and 'modelNamePrefix'.

Start with the command-line handling of --model-name-prefix and the debugModels/debugOperations output for the cpp-qt5-qhttpengine-server generator. Compare why the prefix and modelNamePrefix values are both set, then check how external models are referenced. Done means model and operation prefixes can be configured independently without breaking generated external-model references.