Rename namespace class to {package.name}Package

Conflicts:

	compiler/jet.as.java.psi/src/org/jetbrains/jet/asJava/JavaElementFinder.java

compiler/testData/writeSignature/Nothing.kt

@@ -1,6 +1,6 @@
 fun nothing(): Nothing = throw Exception()
 
-// method: namespace::nothing
+// method: _DefaultPackage::nothing
 // jvm signature:     ()V
 // generic signature: null
 // kotlin signature:  ()Ljet/Nothing;