Expand default constructor warning to cover more cases

Summary

Expand the lint warning added under JDK-8071961 to cover protected classes as well as public ones.

Problem

Besides public classes having default constructors exposed accidentally, a protected class can have its default constructors exposed too as part of the full API of a package.

Solution

Expand the lint warning to cover protected classes with the other conditions staying the same. In detail, a warning is issued for public or protected classes in a named package that has an unqualified export from its module where, for nested classes, all the syntactically enclosing classes must are either public or protected.

Specification

 javac.opt.Xlint.desc.missing-explicit-ctor=\
-    Warn about missing explicit constructors in public classes in exported packages.
+    Warn about missing explicit constructors in public and protected classes in exported packages.