Only prepend remote mixin synced folders when running in remote mode

lib/vagrant/action.rb

@@ -10,6 +10,10 @@ module Vagrant
     # Builtin contains middleware classes that are shipped with Vagrant-core
     # and are thus available to all plugins as a "standard library" of sorts.
     module Builtin
+      module Remote
+        autoload :MixinSyncedFolders, "vagrant/action/builtin/remote/mixin_synced_folders"
+      end
+
       autoload :BoxAdd,    "vagrant/action/builtin/box_add"
       autoload :BoxCheckOutdated, "vagrant/action/builtin/box_check_outdated"
       autoload :BoxRemove, "vagrant/action/builtin/box_remove"

lib/vagrant/action/builtin/mixin_synced_folders.rb

@@ -2,13 +2,13 @@ require "json"
 require "set"
 
 require 'vagrant/util/scoped_hash_override'
-require 'vagrant/action/builtin/remote/mixin_synced_folders'
 
 module Vagrant
   module Action
     module Builtin
       module MixinSyncedFolders
-        prepend Vagrant::Action::Builtin::Remote::MixinSyncedFolders
+        autoload :Remote, "vagrant/action/builtin/remote/mixin_synced_folderse"
+
         include Vagrant::Util::ScopedHashOverride
 
         # This goes over all the registered synced folder types and returns

lib/vagrant/shared_helpers.rb

@@ -269,5 +269,6 @@ module Vagrant
     ->{ Vagrant::MachineIndex::Entry.extend(Vagrant::MachineIndex::Entry::Remote::ClassMethods) },
     ->{ Vagrant::Plugin::V2::SyncedFolder.prepend(Vagrant::Plugin::Remote::SyncedFolder::Remote) },
     ->{ Vagrant::Plugin::V2::Communicator.prepend(Vagrant::Plugin::Remote::Communicator::Remote) },
+    ->{ Vagrant::Action::Builtin::MixinSyncedFolders.prepend(Vagrant::Action::Builtin::Remote::MixinSyncedFolders) },
   ].freeze
 end