Run yarn check --verify-tree as part of the yarn integrity check (#1774)

From the yarn documentation[1]:

`yarn check --integrity` checks that versions and hashed
values of the package contents in the project’s package.json
match those in yarn’s lock file.

That doesn't actually check that the packages are actually
installed in node_modules. To check that we need to run
`yarn check --verify-tree`

[1]: https://yarnpkg.com/lang/en/docs/cli/check/

lib/webpacker/railtie.rb

@@ -30,13 +30,13 @@ class Webpacker::Engine < ::Rails::Engine
   #     - add `config.webpacker.check_yarn_integrity = true`
   initializer "webpacker.yarn_check" do |app|
     if File.exist?("yarn.lock") && Webpacker.config.config_path.exist? && Webpacker.config.check_yarn_integrity?
-      output = `yarn check --integrity 2>&1`
+      output = `yarn check --integrity && yarn check --verify-tree 2>&1`
 
       unless $?.success?
         $stderr.puts "\n\n"
         $stderr.puts "========================================"
         $stderr.puts "  Your Yarn packages are out of date!"
-        $stderr.puts "  Please run `yarn install` to update."
+        $stderr.puts "  Please run `yarn install --check-files` to update."
         $stderr.puts "========================================"
         $stderr.puts "\n\n"
         $stderr.puts "To disable this check, please change `check_yarn_integrity`"