Skip refined method when exporting methods with changed visibility

[jeremyevans]

Previously, attempting to change the visibility of a method in a
singleton class for a class/module that is prepended to and refined
would raise a NoMethodError.

Fixes [Bug #17519]

[ko1]

if the skip type is only REFINED, how about to make skip_refined boolean flag instead of skip_type?

[jeremyevans]

The approach of taking a type is a bit more flexible, in case we want to skip other method types in other cases. However, I can make that change if you prefer. This isn't public API, so we could always change it later if needed.

[nobu]

At least, 0 is a possible valid value as rb_method_type_t.

[jeremyevans]

I originally tried to use -1 instead of 0, but it complained about that :). From the looks of it, using END_OF_VM_METHOD_TYPE_PLACEHOLDER may not be portable, and VM_METHOD_TYPE_REFINED+1 will just break later if we expand the values in rb_method_type_t. So I'll go with @ko1's suggestion.

[fledman]

@jeremyevans
what is your plan for this bugfix?

[jeremyevans]

Let me rebase on master later today and if CI passes I'll merge it.

[fledman]

@jeremyevans
I was running my original set of cases on the just released 2.7.3, and I realized that this fix introduces a new bug.
Now, declaring an unrefined module by itself breaks set_visibility:

RUBY_VERSION

class Thing
  def direction
    'LEFT'
  end
end

Thing.new.singleton_class.send(:private, :direction)

module NeverUsed
  refine Thing do
    def direction
      'UP'
    end
  end
end

Thing.new.singleton_class.send(:private, :direction)

=> "2.7.3"
=> :direction
=> #<Class:#<Thing:0x00007f81500c9580>>
=> #<refinement:Thing@NeverUsed>
NameError (undefined method `direction' for class `#<Class:#<Thing:0x00007f81500b3a00>>')

prepending an empty module avoids this new bug:

module Nothing; end; Thing.prepend(Nothing); Thing.ancestors
=> [Nothing, Thing, Object, PP::ObjectMixin, Kernel, BasicObject]
Thing.new.singleton_class.send(:private, :direction)
=> #<Class:#<Thing:0x00007f9a7a1293e8>>

[fledman]

#4357