Separate check that the implementation doesn't contain excess classes

We need an ability to check that the implementation doesn't contain excess classes.
This check need to be done separately, apart from the check of signatures availability and correctness.

For this check we may give to SigTest
- a set of sig golden files;
- restricted low level packages (java & javax in our case)

SigTest would check that the implementation doesn't contain classes with these packages that are not present in the specified sig files.