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/nestedClassesNonRootPackage.kt

@@ -1,3 +1,4 @@
+// FIR_BLOCKED: LC don't support names with $
 // EXPECTED_ERROR: class J$B is public, should be declared in a file named J$B.java
 
 // FILE: test/JavaClass.java