[Kotlin] Do not use wildcard import
Description
ktlint
cannot auto-correct wildcard import:
➜ openapi-generator git:(kotlin_post_process) ✗ /usr/local/bin/ktlint -F /Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt
/Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt:6:1: Wildcard import (cannot be auto-corrected)
openapi-generator version
Latest master
Command line used for generation
Steps to reproduce
/usr/local/bin/ktlint -F /Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt
Related issues/PRs
https://github.com/OpenAPITools/openapi-generator/pull/1400
Suggest a fix/enhancement
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