Leftover cleanup from when the tool was implemented.

jnativescan uses the 'ClassResolver' class to find both system classes, as well as application classes.

In principle, a class resolver supports both iterating over all classes from that particular source, or looking classes by name. However, the ClassResolver for system classes doesn't support iterating, and the lookup functionality for ClassResolvers from other sources is never used (only iterating).

We should split these two concepts. Have a system class resolver that only supports lookups by name, and then have another concept for 'class file source' (incidentally, such a class already exists), which can be used to iterate over classes from a particular source, without supporting lookups.