chore(all): return concrete types, accept interfaces

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

internal/portforward/get.go

@@ -1,9 +1,5 @@
 package portforward
 
-import "github.com/qdm12/gluetun/internal/portforward/state"
-
-type Getter = state.PortForwardedGetter
-
 func (l *Loop) GetPortForwarded() (port uint16) {
 	return l.state.GetPortForwarded()
 }