Stub generation infrastructure (except for metadata generation) for KAPT+K2

This commit includes the basic Java stubs generation infrastructure and
the corresponding tests. The main entry point is called
Kapt4StubGenerator. Calls to it from production code will be added in a
separate commit.

 #KT-51982

plugins/kapt3/kapt3-compiler/testData/converter/topLevel.kt

@@ -1,3 +1,5 @@
+// WITH_STDLIB
+// FULL_JDK
 package test.another
 
 annotation class Anno(val value: String)