Fix duplicated NestedAnnotationInfo class

[msridhar]

Remind me why we have the duplication between this and library-model/library-model-generator/src/main/java/com/uber/nullaway/libmodel/NestedAnnotationInfo.java ...

Originally posted by @lazaroclapp in #1649 (comment)

I think the right fix here is:

1. Move NestedAnnotationInfo and possibly other related code to a new Gradle module, and have nullaway and library-model depend on it
2. Rather than shipping a separate artifact for this new module, as a build step copy its classes into the main nullaway jar. Possibly the shadow plugin can do this?