Fix module recursive lookup bug

[tompng]

Fixes this bug

module Mod1
  module Sub
  end
end

module Mod2
  include Mod1
end

module Mod3
  include Mod2
  include Sub
  # 'Mod1::Sub' is included, but RDoc thinks unknown constant 'Sub' is included
end

What was wrong with constant lookup implementation

module M1
  module M2
    include A
    include B

    # Constant lookup of C
    # should search under M2, under M1, recursively B, recursively A, recursively Object
    # Was searching under M2, under B, under A, under M1
    include C

    include D
  end
end

Ruby's actual constant priority
document https://docs.ruby-lang.org/ja/latest/doc/spec=2fvariables.html#prio (Sorry, can't find the english version)

# To check constant lookup priority, comment out C=1, C=2, C=3.0, C=3.1, ... in that order
module A; C=4.0; module AA; C=4.1; end; include AA; end
module B; C=3.0; module BB; C=3.1; end; include BB; end
module X; C=:never; end
C=5.0; module O1; C=6.1; end; include O1 # TopLevel ≒ Object
class Object; C=5.1; module O2; C=6.0; end; include O2; end
module M1
  include X
  C=2
  module M2
    C=1
    include A
    include B
    p C
  end
end

[tompng]

Test was wrong.
Actual Mod1::Klass1.ancestors is

[Mod1::Klass1, Mod1::Mod2::Klass0::Mod4, Mod1::Mod2::Mod4, Mod1::Mod3, Mod1::Mod2, Mod1, Object, Kernel, BasicObject]

module Mod1
  module Mod3
  end
  module Mod2
    module Mod3
      module Mod4
      end
    end
    module Mod4
    end
    class Klass0
      module Mod4
        # module Mod5
        # end
        module Mod6
        end
      end
      module Mod5; end
      include Mod4
      include Mod5
      include Mod6
      include Mod1
      include Mod2
      include Mod3
    end
  end
  class Klass1
    include Mod1
    include Mod2
    include Mod3
    include Mod4
    include Klass0::Mod4
  end
end
p Mod1::Klass1.ancestors
# Actual
#=> [Mod1::Klass1, Mod1::Mod2::Klass0::Mod4, Mod1::Mod2::Mod4,      Mod1::Mod3,       Mod1::Mod2, Mod1, Object, Kernel,    BasicObject]
# Test was expecting
#=> [(omitted),    Mod1::Mod2::Klass0::Mod4, Mod1::Mod2::Mod3,Mod4, Mod1::Mod2::Mod3, Mod1::Mod2, Mod1, Object, (omitted), BasicObject]

[tompng]

I found that this part is not correct. should use Module.nesting instead of CodeObject#parent chain.

- while nesting
-   ...
-   nesting = nesting.parent
- end
+ unavailable_data.module_nesting.each do
+   ...
+ end

But I think RDoc::Include and RDoc::Extend does not provide enough information.
This pull request and the original implementation uses CodeObject#parent chain instead of module nesting.

Complicated example

class ::A
  class ::B
    class ::A
      class ::B::C
        # Module.nesting is [::B::C, ::A, ::B, ::A]
        include X
      end
    end
  end
end