Bundler vendors this file and we have some tools to automatically
prepend the `Bundler::` namespace so that the vendored version does not
collide with the stdlib version.

However, due to how methods are defined, it's hard for our vendoring
tool to do the right thing.

I think this makes the code simpler and things easier for us too.

https://github.com/ruby/tsort/commit/7088a7c814