Fix setting method visibility on method wrapped with prepend

	Ignore prepended modules when looking for already defined methods on a
	class to set the visibility on.
	[Fix GH-1834]

	From: Dylan Thacker-Smith <Dylan.Smith@shopify.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62860 b2dd03c8-39d4-4d8f-98ff-823fe69b080e