Properly report bugs to avoid NPEs in FindBugs 3
Allow @VisibleForTesting being called from @Test

 - This makes the report work even when using the -includeTests flag
Improve error messages

 - Report class and field for implicit length
 - Include the names of classes / methods / fields in messages for
    easier correction (specially since when reporting fields we don't have
    a source line)
Fix UnknownNullnessDetector implementation

 - visitMethod was called on each method invocation, not on each method
    definition, so when we asked findbugs to analyze a class, we would get
    incomplete reports, and uncalled public methods would never get reported
 - we also now ignore methods that are inherited, since nullness should be
    defined at the interface level. No matter how hard we try, using @Nonnull
    will not prevent classes in the JRE from calling equals on our object with
    a null value to compare to; and not setting a nullness for those methods
    is ok.