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
Interface methods that were private got both the `default` and `private`
modifiers when using `jvm-default=all` which is not valid.
The stub will now just contain `private` in this case.
This fixes KT-48013.
Pavel Mikhailovskii