Vendored code gen is broken

[xrendan]

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: Shopify/cli-ui#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>'