Use ML completion extension point from 193 platform

- completion-ranking-kotlin jar used to be in the IDEA itself; thanks to new extension it can reside in the plugin from now on
- ML completion is turned off by default
- use `implementation` dependency because `completion-ranking-kotlin` is required during runtime

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/ml/KotlinMLRankingProvider.kt.193

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.completion.ml
+
+import com.completion.ranker.model.kotlin.MLWhiteBox
+import com.intellij.internal.ml.DecisionFunction
+import com.intellij.internal.ml.ModelMetadata
+import com.intellij.internal.ml.completion.CompletionRankingModelBase
+import com.intellij.internal.ml.completion.JarCompletionModelProvider
+import com.intellij.lang.Language
+
+@Suppress("UnstableApiUsage")
+class KotlinMLRankingProvider : JarCompletionModelProvider("Kotlin", "kotlin_features") {
+    override fun createModel(metadata: ModelMetadata): DecisionFunction {
+        return object : CompletionRankingModelBase(metadata) {
+            override fun predict(features: DoubleArray?): Double = MLWhiteBox.makePredict(features)
+        }
+    }
+
+    override fun isLanguageSupported(language: Language): Boolean = language.id.equals("kotlin", ignoreCase = true)
+}

idea/idea-completion/tests/org/jetbrains/kotlin/idea/completion/test/KotlinModelMetadataTest.kt.193

@@ -0,0 +1,14 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.completion.test
+
+import org.jetbrains.kotlin.idea.completion.ml.KotlinMLRankingProvider
+import org.junit.Test
+
+class KotlinModelMetadataTest {
+    @Test
+    fun testMetadataConsistent() = KotlinMLRankingProvider().assertModelMetadataConsistent()
+}