Small tweaks for easier vendoring

[deivid-rodriguez]

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.

[hsbt]

@deivid-rodriguez It's reasonable. Thanks.

[nobu]

Why don't replace TSort in instance methods with self.class?

[hsbt]

@nobu https://github.com/ruby/tsort/pull/3/files#diff-f8dd283765d9907ae3bb5e043d3c9e39e28bcbcbca36c9f420421d724f5d94a1R421 ?

[deivid-rodriguez]

Thank you!

@nobu This patch was enough for our vendoring tool to just work, since previously it would change the code to things like Bundler::TSort.tsort, which... don't really work. So we had to manually nest modules like module Bundler; module TSort; end; end instead of the current style (Module Bundler::TSort; end) so that def TSort.tsort; end would work unchanged.

But... also I think using self.class for instance methods wouldn't work because tsort is mixed into arbitrary classes, yet here we want to delegate to the methods provided by the TSort module, not by the class using tsort.