add missing API to retrieve Kotlin source roots from a CompilerConfiguration

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/KotlinCoreEnvironment.kt

@@ -69,6 +69,7 @@ import org.jetbrains.kotlin.compiler.plugin.ComponentRegistrar
 import org.jetbrains.kotlin.config.CommonConfigurationKeys
 import org.jetbrains.kotlin.config.CompilerConfiguration
 import org.jetbrains.kotlin.config.KotlinSourceRoot
+import org.jetbrains.kotlin.config.kotlinSourceRoots
 import org.jetbrains.kotlin.extensions.ExternalDeclarationsProvider
 import org.jetbrains.kotlin.idea.JetFileType
 import org.jetbrains.kotlin.load.kotlin.JvmVirtualFileFinderFactory
@@ -214,8 +215,7 @@ public class KotlinCoreEnvironment private(
     private fun getSourceRootsCheckingForDuplicates(): Collection<String> {
         val uniqueSourceRoots = Sets.newLinkedHashSet<String>()
 
-        configuration.getList(CommonConfigurationKeys.CONTENT_ROOTS).filterIsInstance<KotlinSourceRoot>().forEach { sourceRoot ->
-            val path = sourceRoot.path
+        configuration.kotlinSourceRoots.forEach { path ->
             if (!uniqueSourceRoots.add(path)) {
                 report(WARNING, "Duplicate source root: $path")
             }

compiler/frontend/src/org/jetbrains/kotlin/config/ContentRoots.kt

@@ -16,6 +16,8 @@
 
 package org.jetbrains.kotlin.config
 
+import java.io.File
+
 public trait ContentRoot
 
 public data class KotlinSourceRoot(public val path: String): ContentRoot
@@ -26,3 +28,7 @@ public fun CompilerConfiguration.addKotlinSourceRoot(source: String) {
 
 public fun CompilerConfiguration.addKotlinSourceRoots(sources: List<String>): Unit = sources.forEach { addKotlinSourceRoot(it) }
 
+public val CompilerConfiguration.kotlinSourceRoots: List<String>
+    get() {
+        return get(CommonConfigurationKeys.CONTENT_ROOTS)?.filterIsInstance<KotlinSourceRoot>()?.map { it.path } ?: emptyList()
+    }