[kotlin][client] improve nonpublic api (#4478)

Author: 4brunu

modules/openapi-generator/src/main/resources/kotlin-client/infrastructure/ApiAbstractions.kt.mustache

@@ -1,6 +1,6 @@
 package {{packageName}}.infrastructure
 
-typealias MultiValueMap = MutableMap<String,List<String>>
+{{#nonPublicApi}}internal {{/nonPublicApi}}typealias MultiValueMap = MutableMap<String,List<String>>
 
 {{#nonPublicApi}}internal {{/nonPublicApi}}fun collectionDelimiter(collectionFormat: String) = when(collectionFormat) {
     "csv" -> ","

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/infrastructure/ApiAbstractions.kt

@@ -1,6 +1,6 @@
 package org.openapitools.client.infrastructure
 
-typealias MultiValueMap = MutableMap<String,List<String>>
+internal typealias MultiValueMap = MutableMap<String,List<String>>
 
 internal fun collectionDelimiter(collectionFormat: String) = when(collectionFormat) {
     "csv" -> ","