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