Vagrant legacy plugin uninstall

[soapy1]

This PR aims to allow vagrant-go users to uninstall legacy Ruby plugins.

• Separates out loading global legacy plugins from local legacy plugins
• Actually loads local plugins (previously they were just ignored)

# Install local plugin
% ./vagrant plugin install vagrant-vbguest --local
  Installing the 'vagrant-vbguest' plugin. This can take a few minutes...
Installed the plugin 'vagrant-vbguest (0.30.0)'!

% cat Vagrantfile
Vagrant.configure("2") do |config|
  config.vm.box = "hashicorp/bionic64"
  config.vbguest.auto_update = false
end

# Local plugin exercised from parsing the config
% ./vagrant status
  Current machine states:
  
  default                   not created (virtualbox)
  
  The environment has not yet been created. Run `vagrant up` to
  create the environment. If a machine is not created, only the
  default provider will be shown. So if a provider is not listed,
  then the machine is not created for that environment.

# Uninstall works
 % ./vagrant plugin uninstall vagrant-vbguest --local
  Uninstalling the 'vagrant-vbguest' plugin...

% ./vagrant plugin list
  No plugins installed.

Fixes: #12864

[soapy1]

I'm not too sure about this approach. If the runner is local then it should be able to load local project plugins. Not sure how to resolve the situation where the runner is remote?