openapi-generator-maven-plugin: a interface defined in the import-mapping is not used by the generated class. It does not implement the interface.

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 you have generated code where X implements Y interface and you now replace the Y interface
using the import-mapping then the result is that X does not implement Y anymore.

X should still implement Y when using import-mapping

openapi-generator version

4.3.1

Command line used for generation

java 11:

false java true false Double=java.math.BigDecimal com.test

						<generateModels>true</generateModels>
						<generateModelTests>false</generateModelTests>
						<generateModelDocumentation>false</generateModelDocumentation>
                                                    <importMappings>Y=com.interface.y</importMappings>
						<generateApis>false</generateApis>
						<generateSupportingFiles>false</generateSupportingFiles>
						<generateApiTests>false</generateApiTests>
						<library>native</library>

						<verbose>false</verbose>
						<validateSpec>true</validateSpec>
						<configOptions>

							<dateLibrary>java8</dateLibrary>
							<useBeanValidation>false</useBeanValidation>

					
						</configOptions>
					</configuration>

Start with the openapi-generator-maven-plugin configuration and reproduce the case using OpenAPI Generator 4.3.1, Java 11, and the shown importMappings setting. Inspect the generated Java class and verify that X implements the mapped Y interface; done means the generated class retains that interface while using the imported type.