This issue serves to track some RFEs and followups required for the `SinceChecker`

1- Currently the `SinceChecker` is called with a `--exclude java.lang.classfile` option as a workaround until the API is finalized. Once ClassFile API is integrated and all `@since` tags have been bumped, it should be removed from the exclude list at it.

2- One additional Idea is identification of redundant tags, say there was a method defined in a type and it has the same `@since` tags as the type. Those tags should be removed (as in JDK-8342653).

3- It would be nice to include unit tests for each case, to avoid any regressions in the future.

4- javac (by design) does not emit warnings when importing preview packages, JEP 12 says:

"For example, suppose a class LargeFile is added to java.io as a normal preview API. With preview features disabled, import java.io.LargeFile; would be a compile-time error; import java.io.*; would not be an error, though any subsequent use of the simple type name LargeFile would be, such as new LargeFile() or LargeFile.MAX_SIZE."

Because of this, the compiler does not recognize if a package was annotated with `@PreviewFeature ` and the test may may report false positives in the future, we should find a way to programmatically know if a package is currently in preview of not.