Namespaced type with nested name reopens type in parent namespace

[straight-shoota]

The following code demonstrates the faulty behaviour:

class Foo::Bar
end

module Baz
  class Foo::Bar
    def baz
    end
  end
end

Baz::Foo::Bar # Error: undefined constant Baz::Foo::Bar

::Foo::Bar.new.baz # works!

Apparently, class Foo::Bar inside the Baz namespace is not actually placed in that namespace. It reopens ::Foo::Bar instead of defining Baz::Foo::Bar.

Originally reported in #11180