Move @AnnotatedFor check to BaseTypeChecker

[thisisalexandercook]

This is a follow-up to the @AnnotatedFor refactor in eisop#1331. It keeps the shared @AnnotatedFor scope computation, but moves ownership of the recursive/cached scope check out of AnnotatedTypeFactory and into BaseTypeChecker.

This was previously blocked by QualifierDefaults calling into BaseTypeChecker before normal checker/type-factory initialization was complete, as discussed here. The concrete failure path comes from stub/ajava parsing during type factory postInit():

GenericAnnotatedTypeFactory.postInit()
  -> parseAnnotationFiles()
  -> stub/ajava parser asks the type factory for a defaulted type
  -> QualifierDefaults.annotate(...)
  -> QualifierDefaults.applyConservativeDefaults(...)
  -> checker.isElementAnnotatedForThisCheckerOrUpstreamChecker(...)
  -> BaseTypeChecker.getTypeFactory()

At that point, routing through the checker can cause an NPE because getTypeFactory() depends on the visitor, which is not available yet.

This PR avoids that by skipping conservative unchecked-code @AnnotatedFor lookup while stub/ajava parsing is active. This does not change the existing policy for stub-file elements: QualifierDefaults currently treats stub files as checked code, see comment here. That existing block handles elements after they are known to come from parsed stub files. The new guard handles the earlier phase where annotation-file metadata is still being parsed.

Most stubs, including annotated-JDK class stubs, are only indexed during initialization and parsed lazily later, when an annotation lookup reaches that JDK class. The risky cases are the annotation files parsed eagerly during postInit(), such as checker @StubFiles, command-line stubs, ajava files, and annotated-JDK package-info.java files.

[aosen-xiong]

Thanks!