Move infrastructure APIs under a specific package

[fmbenhassine]

As of v5.2, all public APIs of a given module are under a specific package (org.springframework.batch.core in spring-batch-core, org.springframework.batch.test in spring-batch-test, etc) except spring-batch-infrastructure.

Infrastructure APIs are under org.springframework.batch.* which makes them difficult to distinguish from other APIs. For example, it is not clear if org.springframework.batch.item.ExecutionContext is an infrastructure concept or a core concept. And by the way, ExecutionContext is currently in the infrastructure module while it should be in the core module (every step has its execution context and not only item oriented steps).