chore(all): return concrete types, accept interfaces

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

internal/provider/privateinternetaccess/updater/updater.go

@@ -8,10 +8,6 @@ type Updater struct {
 	client *http.Client
 }
 
-type Warner interface {
-	Warn(s string)
-}
-
 func New(client *http.Client) *Updater {
 	return &Updater{
 		client: client,