Allow NotNull and Nullable on package declarations

[spnda]

When @NotNull and @Nullable are used on a package declaration in a package-info.class, all fields in all classes should be not-nullable or nullable by default, respectively. Frameworks like Spring have had these annotations for a while (@NonNullFields and @NonNullApi, and would therefore not require any new implementation for many static analyzers). It might also be a bit better to make these new annotations, but that's up to you, reviewers.

[amaembo]

Duplicated by #18
This problem is much more complex than just adding a new target and two lines of documentation. There are many questions to address, e.g.:

• Does it apply to method parameters? What if it's necessary?
• What about inheritance? What if we declare interface with a method String getString() in @NotNull package, then implement it inside the @Nullable package? Which resulting nullability should be used?
• Does it apply to declared type parameters? What about type arguments? What if we declare interface X<T> {} in @NotNull package? Will all instantiations of T be assumed as @NotNull? What if we instantiate it inside the @Nullable package?
• IntelliJ IDEA provides automatic instrumentation via annotation processor that turns @NotNull method parameters and return values into runtime assertions. It's quite difficult to implement this feature if you have @NotNull in package declaration which is a separate file (and can be even separately compiled).

I'm closing this, as we already have #18 to track the same problem.