Vendored code gen is broken

If creating a new cli app using the vendored method, cli-ui is vendored without it's own vendor directory. This issue was solved for the bundled gem here: https://github.com/Shopify/cli-ui/issues/418

Reproduction:

cli-kit new myapp
# (select vendor)
myapp/exe/myapp

/Users/brendansamek/startup/reactor/reactor/vendor/deps/cli-ui/lib/cli/ui/stdout_router.rb:6:in 'Kernel#require_relative': cannot load such file -- /Users/brendansamek/startup/reactor/reactor/vendor/deps/cli-ui/vendor/reentrant_mutex (LoadError)
	from /Users/brendansamek/startup/reactor/reactor/vendor/deps/cli-ui/lib/cli/ui/stdout_router.rb:6:in '<top (required)>'
	from /Users/brendansamek/startup/reactor/reactor/vendor/deps/cli-ui/lib/cli/ui.rb:359:in 'Kernel#require'
	from /Users/brendansamek/startup/reactor/reactor/vendor/deps/cli-ui/lib/cli/ui.rb:359:in '<top (required)>'
	from /Users/brendansamek/startup/reactor/reactor/lib/reactor.rb:1:in 'Kernel#require'
	from /Users/brendansamek/startup/reactor/reactor/lib/reactor.rb:1:in '<top (required)>'
	from exe/reactor:14:in 'Kernel#require'
	from exe/reactor:14:in '<main>'

Run cli-kit new myapp, select the vendor method, and reproduce the LoadError when running myapp/exe/myapp. Start with vendor/deps/cli-ui/lib/cli/ui/stdout_router.rb and compare the vendored path with the bundled-gem fix referenced in cli-ui issue 418. Done means a newly generated vendored app runs without the missing reentrant_mutex error.