Finish up the job of skipping symlinks when linting.
Finish up the job of skipping symlinks when linting.

I wasn't handling the find-files-under-directory case before!

Auditors: benkraft

Test Plan:
   % ka-lint -v tools
   ... skipping directory /home/csilvers/khan/webapp/tools/progress-icons: is a symlink
   ... skipping file /home/csilvers/khan/webapp/tools/sync_tap_data.py: is a symlink
   [etc]

   % ka-lint -v translations/tools
   ...
   running extra linter /home/csilvers/khan/webapp/tools/runlint.sh on these files: ['/home/csilvers/khan/webapp/translations/tools/3play_media.py', ...]

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter