Now, all logic for parsing loading `astubx` files is consolidated into
the `LibraryModelsHandler.ExternalStubxLibraryModels` class and is
uniform for `astubx` files generated via JarInfer or from source code,
which will ease future bug fixes and changes.

We copied over the logic from `InferredJARModelsHandler` for loading
Android SDK models if present. We also update the logic for
`ExternalStubxLibraryModels.nonNullParameters()` and
`ExternalStubxLibraryModels.nullableReturns()` to pull in all the
relevant info from the parsed `astubx` file.

Fixes #1072

Fixes #1093 

We just missed a case.

Address
#1097 (comment)

In Java 21+, when a `switch` statement contains `case null`, the
expression being switched on is allowed to be `null`.

Fixes #930

Fixes #1101

#1105)

Fixes #1110 

We can see how this goes for some time before making it a required job.

…rInfer (#1114)

Our previous code would crash on nested generic types or when multiple
unbounded wildcard type arguments were passed consecutively.

This is in preparation to attempt to fix #1091. Just a refactoring, no
behavior changes.

Fixes #1113 

We weren't calling the right API to check the nullability of a varargs
argument when checking valid overriding. Most of this PR is tests to
check handling of most of the cases.

Fixes #574 

We add a new flag `OnlyNullMarked` to indicate that NullAway can only
check code that is `@NullMarked`, and hence it is ok to omit the
(previously required) `AnnotatedPackages` flag. We currently require
_exactly_ one of either `AnnotatedPackages` or `OnlyNullMarked` to be
passed in; if both flags are omitted or both flags are passed, we fail.

As JSpecify and `@NullMarked` become more widely adopted, we may change
the policy to allow neither flag to be passed (as NullAway already
checks `@NullMarked` code out of the box). But for now, we require one
of the flags to be passed, to avoid confusion.

This PR adds support for static fields in
`@EnsureNonnull`,`EnsureNonnullIf`,`@RequiresNonnull` annotations.
Currently the following code will throw validation errors (as well as
the annotation handlers are unable to handle static fields). However,
after this change, static fields for all three annotations are supported

```
class Foo {
  @nullable static Item staticNullableItem;

  @EnsuresNonNull("staticNullableItem")
  public static void initializeStaticField() {
    staticNullableItem = new Item();
  }

  @RequiresNonNull("staticNullableItem")
  public static void useStaticField() {
    staticNullableItem.call();
  }

  @EnsuresNonNullIf(value="staticNullableItem", result=true)
  public static boolean hasStaticNullableItem() {
    return staticNullableItem != null;
  }
}
```
Fixes #431

---------

Co-authored-by: Manu Sridharan <[email protected]>