chore(constants): internal/constants/providers

- New package to avoid package import cycles

internal/configuration/sources/env/provider.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/constants/providers"
 )
 
 func (r *Reader) readProvider(vpnType string) (provider settings.Provider, err error) {
@@ -35,11 +36,11 @@ func (r *Reader) readVPNServiceProvider(vpnType string) (vpnProviderPtr *string)
 	switch {
 	case vpnType != constants.Wireguard &&
 		os.Getenv("OPENVPN_CUSTOM_CONFIG") != "": // retro compatibility
-		return stringPtr(constants.Custom)
+		return stringPtr(providers.Custom)
 	case s == "":
 		return nil
 	case s == "pia": // retro compatibility
-		return stringPtr(constants.PrivateInternetAccess)
+		return stringPtr(providers.PrivateInternetAccess)
 	}
 	return stringPtr(s)
 }