Allow missing method implementation in Ruby code

[soutaro]

Having multiple module and class declarations causes a problem of undefined method ____'onalias_methodcalls. This is because the hook installation happens on:end` event of TracePoint.

module SuperLibrary
  # defines classes of the library
  class SomeClass
  end
end                        # Hook install happens here, but no `foo` method defined yet. 💣 💥 

module SuperLibrary
  # The library itself has methods
  def foo()
    # ...
  end
end

To avoid this error, it now does method existence check and hook definition duplication check.