Fix path lookup when ancestor finds type with same name as current scope

[HertzDevil]

Fixes #4086.

class Foo
  class Bar
    class Baz < Foo
      def self.foo
        # `Baz` is now tried in the following order:
        #
        # * `Baz` relative to `::Foo::Bar::Baz` (direct namespace child)
        # * `::Foo::Bar::Baz` (current scope)
        # * `Baz` relative to `::Foo` (ancestors)
        # * `Baz` relative to `::Foo::Bar` (enclosing namespace)
        #
        # this only applies to the first path item; subsequent items must
        # descend along namespaces so `Baz::Baz` is still undefined
        Baz
      end
    end
  end

  class Baz
  end
end

Foo::Bar::Baz.foo # => Foo::Bar::Baz

This behaviour is consistent with Ruby and C++.

The path lookup rules are unchanged if the path's first item and the current scope's last path item do not match.