Add Gradle plugin marker artifacts for the Gradle plugins.

This allows for the Gradle plugins DSL to resolve the plugins from
a custom repository.
https://docs.gradle.org/current/userguide/plugins.html#sec:plugin_markers

libraries/tools/gradle-tools/pluginMarkers.gradle

@@ -0,0 +1,47 @@
+ext.pluginMarkerProject = { String pluginProjectId ->
+    evaluationDependsOn(":$pluginProjectId")
+
+    def pluginProject = findProject(":$pluginProjectId")
+
+    repositories.addAll(pluginProject.repositories.toList())
+
+    apply plugin: 'java'
+
+    dependencies {
+        compile pluginProject
+    }
+
+    configurations.archives.artifacts.clear()
+
+    configurePublishing(project)
+
+    task emptyJar(type: Jar)
+
+    version = project.version + (findProperty('marker_version_suffix') ?: "")
+
+    for (pluginId in pluginProject.pluginBundle.plugins*.id) {
+        def artifactName = "${pluginId}.gradle.plugin"
+
+        artifacts {
+            ['', 'javadoc', 'sources'].forEach { theClassifier ->
+                archives(emptyJar) {
+                    group = pluginId
+                    name = artifactName
+                    classifier theClassifier
+                }
+            }
+        }
+
+        def setUpSeparateArtifact = {
+            addFilter(pluginId) { artifact, file -> artifact.name == artifactName }
+            def pom = pom(pluginId)
+            pom.groupId = pluginId
+            pom.artifactId = "${pluginId}.gradle.plugin"
+        }
+
+        configure(uploadArchives.repositories.mavenDeployer, setUpSeparateArtifact)
+        configure(install.repositories.mavenInstaller, setUpSeparateArtifact)
+    }
+
+    publish.dependsOn uploadArchives
+}

libraries/tools/gradle-tools/settings.gradle

@@ -4,7 +4,12 @@ include ':kotlin-gradle-plugin-api',
         ':kotlin-allopen', 
         ':kotlin-noarg',
         ':kotlin-sam-with-receiver',
-        ':kotlin-gradle-subplugin-example'
+        ':kotlin-gradle-subplugin-example',
+
+        // plugin markers:
+        ':kotlin-gradle-plugin:plugin-marker',
+        ':kotlin-allopen:plugin-marker',
+        ':kotlin-noarg:plugin-marker'
 
 project(':kotlin-gradle-plugin-api').projectDir = file("../kotlin-gradle-plugin-api")
 project(':kotlin-gradle-plugin').projectDir = file("../kotlin-gradle-plugin")
@@ -12,4 +17,10 @@ project(':kotlin-gradle-plugin-integration-tests').projectDir = file("../kotlin-
 project(':kotlin-allopen').projectDir = file("../kotlin-allopen")
 project(':kotlin-noarg').projectDir = file("../kotlin-noarg")
 project(':kotlin-sam-with-receiver').projectDir = file("../kotlin-sam-with-receiver")
-project(':kotlin-gradle-subplugin-example').projectDir = file("../../examples/kotlin-gradle-subplugin-example")
+project(':kotlin-gradle-subplugin-example').projectDir = file("../../examples/kotlin-gradle-subplugin-example")
+
+// plugin markers:
+project(':kotlin-gradle-plugin:plugin-marker').projectDir = file("../kotlin-gradle-plugin/plugin-marker")
+project(':kotlin-allopen:plugin-marker').projectDir = file("../kotlin-allopen/plugin-marker")
+project(':kotlin-noarg:plugin-marker').projectDir = file("../kotlin-noarg/plugin-marker")
+

libraries/tools/kotlin-allopen/plugin-marker/build.gradle

@@ -0,0 +1,3 @@
+apply from: "$rootDir/pluginMarkers.gradle"
+
+pluginMarkerProject('kotlin-allopen')

libraries/tools/kotlin-gradle-plugin/plugin-marker/build.gradle

@@ -0,0 +1,3 @@
+apply from: "$rootDir/pluginMarkers.gradle"
+
+pluginMarkerProject('kotlin-gradle-plugin')

libraries/tools/kotlin-noarg/plugin-marker/build.gradle

@@ -0,0 +1,3 @@
+apply from: "$rootDir/pluginMarkers.gradle"
+
+pluginMarkerProject('kotlin-noarg')