chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/vpn/wireguard.go

@@ -5,8 +5,6 @@ import (
 	"fmt"
 
 	"github.com/qdm12/gluetun/internal/configuration/settings"
-	"github.com/qdm12/gluetun/internal/firewall"
-	"github.com/qdm12/gluetun/internal/netlink"
 	"github.com/qdm12/gluetun/internal/provider"
 	"github.com/qdm12/gluetun/internal/provider/utils"
 	"github.com/qdm12/gluetun/internal/wireguard"
@@ -14,10 +12,10 @@ import (
 
 // setupWireguard sets Wireguard up using the configurators and settings given.
 // It returns a serverName for port forwarding (PIA) and an error if it fails.
-func setupWireguard(ctx context.Context, netlinker netlink.NetLinker,
-	fw firewall.VPNConnectionSetter, providerConf provider.Provider,
+func setupWireguard(ctx context.Context, netlinker NetLinker,
+	fw Firewall, providerConf provider.Provider,
 	settings settings.VPN, logger wireguard.Logger) (
-	wireguarder wireguard.Wireguarder, serverName string, err error) {
+	wireguarder *wireguard.Wireguard, serverName string, err error) {
 	connection, err := providerConf.GetConnection(settings.Provider.ServerSelection)
 	if err != nil {
 		return nil, "", fmt.Errorf("failed finding a VPN server: %w", err)