Rename package jet -> kotlin in ant

org.jetbrains.jet.buildtools.ant -> org.jetbrains.kotlin.ant

The old antlib.xml is kept for migration purposes

docs/confluence.jetbrains.com/Kotlin/40702623_Kotlin+Build+Tools.confluence

@@ -8,7 +8,7 @@ One way to define Ant's {{*<kotlinc>*}} task is by using your local Kotlin setup
 
 {code:xml}
 <property environment="env"/>
-<taskdef resource    = "org/jetbrains/jet/buildtools/ant/antlib.xml">
+<taskdef resource    = "org/jetbrains/kotlin/ant/antlib.xml">
     <classpath>
         <fileset dir = "${env.KOTLIN_HOME}/lib" includes = "*.jar"/>
     </classpath>