Setup conf2ScopeMappings for 'api' configuration with 'maven' plugin

The 'java' plugin doesn't expect the 'api' configuration to exist (it
is only introduced by the 'java-library' plugin) and doesn't map it
to the compile scope when publishing with the legacy 'maven' plugin.

This commit adds the missing conf2ScopeMapping for 'api' -> 'compile'

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt

@@ -7,6 +7,7 @@ import groovy.lang.Closure
 import org.gradle.api.*
 import org.gradle.api.artifacts.ExternalDependency
 import org.gradle.api.artifacts.MutableVersionConstraint
+import org.gradle.api.artifacts.maven.Conf2ScopeMappingContainer
 import org.gradle.api.artifacts.maven.MavenResolver
 import org.gradle.api.attributes.Usage
 import org.gradle.api.file.FileCollection
@@ -16,6 +17,7 @@ import org.gradle.api.logging.Logging
 import org.gradle.api.plugins.InvalidPluginException
 import org.gradle.api.plugins.JavaPlugin
 import org.gradle.api.plugins.JavaPluginConvention
+import org.gradle.api.plugins.MavenPluginConvention
 import org.gradle.api.publish.PublishingExtension
 import org.gradle.api.publish.maven.MavenPublication
 import org.gradle.api.tasks.CompileClasspathNormalizer
@@ -383,6 +385,10 @@ internal abstract class AbstractKotlinPlugin(
                     }
                 }
             }
+
+            // Setup conf2ScopeMappings so that the API dependencies are wriiten with the compile scope in the POMs in case of 'java' plugin
+            project.convention.getPlugin(MavenPluginConvention::class.java)
+                .conf2ScopeMappings.addMapping(0, project.configurations.getByName("api"), Conf2ScopeMappingContainer.COMPILE)
         }
     }