[java] Make "unresolved symbol" semantic warnings trace or debug logs

[oowekyala]

Is your feature request related to a problem? Please describe.

ee0e751 turns on logging of unresolved types (ref #1371). These logs are reported as warnings, but they're very noisy if you haven't configured your classpath properly.

Describe the solution you'd like
Ideally they would all be output to the TRACE (or DEBUG) level (ie only visible with --debug), and without --debug a single summary should be printed (eg WARN: Some symbols were unresolved, please run with --debug to see which ones and fix your --aux-classpath)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

• related to [core] Error reporting vs logging #3816