[rubygems/rubygems] Vendor tsort into rubygems

So that it loads a consistent version of the library and `rubygems` is
never affected by gem activation conflicts related to `tsort`.

Getting CI green required updating one `bundler` spec, because `tsort`
is no longer loaded by `bundle clean` until after `BUNDLE_PATH` has been
changed, so to ensure it is found, it needs to be installed under
`BUNDLE_PATH` as well (which will be different from the global system
path on Bundler 3, meaning installing `tsort` to the global system path
is not enough there). This spec workaround can be removed once we also
vendor `tsort` inside `bundler`.

d326880999

lib/rubygems/request_set.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require 'tsort'
+require_relative 'tsort'
 
 ##
 # A RequestSet groups a request to activate a set of dependencies.
@@ -15,7 +15,7 @@ require 'tsort'
 #   #=> ["nokogiri-1.6.0", "mini_portile-0.5.1", "pg-0.17.0"]
 
 class Gem::RequestSet
-  include TSort
+  include Gem::TSort
 
   ##
   # Array of gems to install even if already installed