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

Problem
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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
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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
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     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.