[Kotlin] Bug generating model import with --import-mappings

Description

Kotlin generator generates faulty imports for models imported with --import-mappings. When I specify fully qualified class name to import via --import-mappings config, the generator prepends package from --model-package property. For the cmd below it generates this model:

package io.swagger.petstore.model

import io.swagger.petstore.model.Tag
import io.swagger.petstore.model.org.example.model.Category

/**
 * 
 * @param id 
 * @param category 
 * @param name 
 * @param photoUrls 
 * @param tags 
 * @param status pet status in the store
 */
data class Pet (
    val name: kotlin.String,
    val photoUrls: kotlin.Array,
    val id: kotlin.Long? = null,
    val category: org.example.model.Category? = null,
    val tags: kotlin.Array? = null,
    /* pet status in the store */
    val status: Pet.Status? = null
)
//..rest of the file

Swagger-codegen version

swagger-codegen-cli-2.4.12

Swagger declaration file content or url

http://petstore.swagger.io/v2/swagger.json

Command line used for generation

java -jar swagger-codegen-cli.jar generate -Dmodels \
  -i http://petstore.swagger.io/v2/swagger.json \
  -l kotlin-server \
  -o generated \
  --model-package io.swagger.petstore.model \
  --import-mappings Category=org.example.model.Category

Suggest a fix/enhancement

I believe we shouldn't add imports for classes specified in --import-mappings config, so the possible workaround would be adding && !importMapping.values().contains(type) to AbstractKotlinCodegen.java#L538

Start with AbstractKotlinCodegen.java around line 538 and reproduce the issue using the provided swagger-codegen-cli command with --import-mappings Category=org.example.model.Category. Check how the Kotlin generator builds imports for mapped models. Done means the generated model does not prepend --model-package to the fully qualified mapped class name.