gradle-plugin: Don't use Project.getTasksByName method

Call of Project.getTasksByName in KonanPluing.apply causes subproject
evaluation. So in such a build script the plugin 'foo' will not be
applied to the subprojects:

subprojects {
    apply 'konan'
    apply 'foo'
}

This patch uses methods available via Project.tasks property to
work with tasks without causing subproject evaluation.

Related issue: KT-19916

tools/kotlin-native-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KonanPlugin.kt

@@ -215,23 +215,17 @@ class KonanPlugin @Inject constructor(private val registry: ToolingModelBuilderR
         }.getProperty("default-konan-version") ?: throw RuntimeException("Cannot read the default compiler version")
     }
 
+    /** Looks for task with given name in the given project. Throws [UnknownTaskException] if there's not such task. */
+    private fun Project.getTask(name: String): Task = tasks.getByPath(name)
+
     /**
      * Looks for task with given name in the given project.
      * If such task isn't found, will create it. Returns created/found task.
      */
-    private fun Project.getTask(name: String): Task = getTasksByName(name, false).single()
-
-    private fun Project.getOrCreateTask(name: String): Task {
-        val tasks = getTasksByName(name, false)
-        assert(tasks.size <= 1)
-        return if (tasks.isEmpty()) {
-            this.tasks.create(name, DefaultTask::class.java)
-        } else {
-            tasks.single()
-        }
+    private fun Project.getOrCreateTask(name: String): Task = with(tasks) {
+        findByPath(name) ?: create(name, DefaultTask::class.java)
     }
 
-    // TODO: Create default config? what about test sources?
     override fun apply(project: Project?) {
         if (project == null) { return }
         registry.register(KonanToolingModelBuilder)

tools/kotlin-native-gradle-plugin/src/test/groovy/org/jetbrains/kotlin/gradle/plugin/test/KonanProject.groovy

@@ -18,6 +18,7 @@ class KonanProject {
 
     File         buildFile
     File         propertiesFile
+    File         settingsFile
 
     Set<File>    srcFiles = []
 
@@ -77,12 +78,18 @@ class KonanProject {
         createSubDir("src", "main", "kotlin")
     }
 
-    /** Generates a build.gradle file in root project directory with the given content. */
+    /** Generates a build.gradle file in the root project directory with the given content. */
     File generateBuildFile(String content) {
         buildFile = createFile(projectPath, "build.gradle", content)
         return buildFile
     }
 
+    /** Generates a settings.gradle file in the root project directory with the given content. */
+    File generateSettingsFile(String content) {
+        settingsFile = createFile(projectPath, "settings.gradle", content)
+        return settingsFile
+    }
+
     /**
      * Generates a build.gradle file in root project directory with the default content (see below)
      * and fills the compilationTasks array.
@@ -197,6 +204,7 @@ class KonanProject {
             generateFolders()
             generateBuildFile()
             generatePropertiesFile(konanHome)
+            generateSettingsFile("")
         }
         return result
     }

tools/kotlin-native-gradle-plugin/src/test/groovy/org/jetbrains/kotlin/gradle/plugin/test/RegressionSpecification.groovy

@@ -0,0 +1,38 @@
+package org.jetbrains.kotlin.gradle.plugin.test
+
+import org.gradle.testkit.runner.TaskOutcome
+
+class RegressionSpecification extends BaseKonanSpecification {
+
+    def 'KT-19916'() {
+        when:
+            def project = KonanProject.createEmpty(getProjectDirectory()) { KonanProject prj ->
+                prj.generateSettingsFile("include ':subproject'")
+                def subprojectDir = prj.projectPath.resolve("subproject").toFile()
+                subprojectDir.mkdirs()
+                subprojectDir.toPath().resolve("build.gradle").write("""
+                    dependencies {
+                        libs gradleApi()
+                    }
+                """.stripIndent())
+
+                prj.buildFile.append("""
+                    subprojects {
+                        apply plugin: 'konan'
+                        apply plugin: Foo
+                    }
+                    
+                    class Foo implements Plugin<Project> {
+                        void apply(Project project) {
+                            project.configurations.maybeCreate("libs")
+                        }
+                    }
+                """.stripIndent())
+            }
+
+            def result = project.createRunner().withArguments('tasks').build()
+        then:
+            result.task(':tasks').outcome == TaskOutcome.SUCCESS
+    }
+
+}