RFR: 8178017: JDK 9 change to symlink handling causes misleading class.public.should.be.in.file diagnostic

[Jonathan Gibbons]

Review for relatively simple fix to revert a change in behavior in JDK 9:

JBS: https://bugs.openjdk.java.net/browse/JDK-8178017
Webrev: http://cr.openjdk.java.net/~jjg/8178017/webrev/

-- Jon