Rename package jet -> kotlin in ant

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

The old antlib.xml is kept for migration purposes

compiler/testData/integration/ant/jvm/externalAnnotations/root1/a/j/annotations.xml

@@ -2,4 +2,4 @@
   <item name='j.Java java.lang.String f()'>
     <annotation name='org.jetbrains.annotations.NotNull'/>
   </item>
-</root>
+</root>

compiler/testData/integration/ant/jvm/externalAnnotations/root1/b/j/annotations.xml

@@ -2,4 +2,4 @@
   <item name='j.Java java.lang.String f2()'>
     <annotation name='org.jetbrains.annotations.NotNull'/>
   </item>
-</root>
+</root>