Capture package-level annotations from dependency types via a new `JavaType.Package`

[timtebeek]

Background

The Java LST type model has no representation for packages. JavaType.Class.owningClass only stores enclosing FullyQualified (i.e. class-like) symbols (ReloadableJava21TypeMapping.java:229-231), and the javac/reflection type mappings never read PackageSymbol.getAnnotationMirrors() or Package.getDeclaredAnnotations(). As a result, package-level annotations declared in a dependency's package-info.class are not present anywhere in the LST.

This blocks nullability inference recipes that need to honor:

• JSpecify @NullMarked / @NullUnmarked
• JSR-305 @ParametersAreNonnullByDefault / @FieldsAreNonnullByDefault
• FindBugs/SpotBugs default-scope annotations
• any other annotation that propagates from package to class to member

As a related symptom, even within source: FindMissingTypesTest.java:172-175 asserts that the type of @NullMarked on a package-info.java resolves to JavaType.Unknown — i.e. package annotations don't resolve their types even when the annotation source is the current project.

Proposal

Introduce JavaType.Package implements JavaType with fullyQualifiedName and annotations: FullyQualified[], accessed via a new @Nullable Package pkg field on JavaType.Class (plus a default getPackage() accessor on FullyQualified).

Scope

• New JavaType.Package type, with constructor/unsafeSet and the @JsonTypeInfo machinery.
• Add pkg field + getPackage() to JavaType.Class; extend unsafeSet overloads.
• Populate from javac (ReloadableJava{8,11,17,21,25}TypeMapping), reflection (JavaReflectionTypeMapping — including a forced load of package-info.class so Package.getDeclaredAnnotations() returns non-empty), Kotlin (KotlinTypeMapping, KotlinIrTypeMapping), Scala (ScalaTypeMapping), and the source-side parser (ReloadableJava{N}ParserVisitor, which already collects getPackageAnnotations() syntactically).
• Signature builder: add packageSignature(Object) to JavaTypeSignatureBuilder and all implementations; signature "<package> <fqn>" to keep it disjoint from class signatures.
• Visitor / RPC: add visitPackage to JavaTypeVisitor, wire JavaTypeSender / JavaTypeReceiver.
• Flip FindMissingTypesTest:172-175 from "expect Unknown" to "expect annotation type resolved".
• Convenience: TypeUtils.findInheritedAnnotation(JavaType, String) that walks type-use → declaration → enclosing class → package.

Non-goals

• Module annotations (module-info) — separate problem.
• Implementing nullability inference itself — this issue only unblocks it.

Compatibility

New optional field, additive. Existing recipes see no behavior change. RPC consumers running an older JavaTypeReceiver will fail on Package payloads — roll out the receiver change first, then the sender.

Acceptance criteria

• A class loaded from a JAR whose package-info.class is annotated with @org.jspecify.annotations.NullMarked exposes that annotation via cls.getPackage().getAnnotations().
• Same for reflection-mapped classes.
• A J.Package.annotations[0].getType() in a parsed package-info.java is no longer JavaType.Unknown.
• Round-trips correctly through RPC.