builder.proxmox: make ProxmoxURL *url.URL field unexported

the only exported field we/users care about is ProxmoxURLRaw

builder/proxmox/builder.go

@@ -41,7 +41,7 @@ func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (pack
 	tlsConfig := &tls.Config{
 		InsecureSkipVerify: b.config.SkipCertValidation,
 	}
-	b.proxmoxClient, err = proxmox.NewClient(b.config.ProxmoxURL.String(), nil, tlsConfig)
+	b.proxmoxClient, err = proxmox.NewClient(b.config.proxmoxURL.String(), nil, tlsConfig)
 	if err != nil {
 		return nil, err
 	}

builder/proxmox/config.go

@@ -27,7 +27,7 @@ type Config struct {
 	Comm                   communicator.Config `mapstructure:",squash"`
 
 	ProxmoxURLRaw      string `mapstructure:"proxmox_url"`
-	ProxmoxURL         *url.URL
+	proxmoxURL         *url.URL
 	SkipCertValidation bool   `mapstructure:"insecure_skip_tls_verify"`
 	Username           string `mapstructure:"username"`
 	Password           string `mapstructure:"password"`
@@ -184,7 +184,7 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	if c.ProxmoxURLRaw == "" {
 		errs = packer.MultiErrorAppend(errs, errors.New("proxmox_url must be specified"))
 	}
-	if c.ProxmoxURL, err = url.Parse(c.ProxmoxURLRaw); err != nil {
+	if c.proxmoxURL, err = url.Parse(c.ProxmoxURLRaw); err != nil {
 		errs = packer.MultiErrorAppend(errs, errors.New(fmt.Sprintf("Could not parse proxmox_url: %s", err)))
 	}
 	if c.ISOFile == "" {