Gradle build fixes & improvements:

* Fix Java not compiled in kotlin-gradle-plugin Kotlin source set
* Fix the integration tests running under Java 6
* Fix kotlin-gradle-subplugin-example not being installed -- needed
  for a test
* Fix Android tests being always excluded
* Move versions out of `build.gradle` (x2) to a common `versions.gradle`
* Move JDK, dist and bootstrap to `commonConfiguration.gradle`
* Build `kotlin-gradle-plugin` with JDK_18

libraries/tools/gradle-tools/build.gradle

@@ -1,7 +1,6 @@
 buildscript {
-    ext.kotlin_version = findProperty("deployVersion") ?: "1.1-SNAPSHOT"
-    ext.kotlin_language_version = "1.1"
-    ext.kotlin_gradle_plugin_version = findProperty("kotlinGradlePluginVersion") ?: "1.1.1"
+    ext.kotlin_root = findProperty("kotlinRoot") ?: "${rootProject.projectDir}/../../.."
+    apply from: "${kotlin_root}/libraries/versions.gradle"
 
     repositories {
         mavenCentral()
@@ -11,19 +10,8 @@ buildscript {
     }
 }
 
-ext.kotlin_root = findProperty("kotlinRoot") ?: "${rootProject.projectDir}/../../.."
-
 apply from: "${kotlin_root}/libraries/commonConfiguration.gradle"
 
-ext {
-    JDK_16 = jdkPath("1.6")
-    JDK_17 = jdkPath("1.7")
-    JDK_18 = jdkPath("1.8")
-    distDir = file("${kotlin_root}/dist")
-    distLibDir = file("${kotlin_root}/dist/kotlinc/lib")
-    bootstrapCompilerFile = file("$distDir/kotlin-compiler-for-maven.jar")
-}
-
 allprojects {
     group = 'org.jetbrains.kotlin'
     version = "$kotlin_version"